Compatibility with GnuPG 1.x and 2.x, auto-detect GnuPG version

* aptly can sign and verify without issues with GnuPG 1.x and 2.x
* aptly auto-detects GnuPG version and adapts accordingly
* aptly automatically finds suitable GnuPG version

Majority of the work was to get unit-tests which can work with GnuPG 1.x & 2.x.
Locally I've verified that aptly supports GnuPG 1.4.x & 2.2.x. Travis CI
environment is based on trusty, so it runs gpg2 tests with GnuPG 2.0.x.

Configuration parameter gpgProvider now supports three values for GnuPG:

* gpg (same as before, default): use GnuPG 1.x if available (checks gpg, gpg1),
otherwise uses GnuPG 2.x; for aptly users who already have GnuPG 1.x
environment (as it was the only supported version) nothing should change; new
users might start with GnuPG 2.x if that's their installed version

* gpg1 looks for GnuPG 1.x only, fails otherwise

* gpg2 looks for GnuPG 2.x only, fails otherwise

pgp/gnupg_finder.go

@@ -0,0 +1,149 @@
+package pgp
+
+import (
+	"errors"
+	"os/exec"
+	"regexp"
+	"strings"
+)
+
+// GPGVersion stores discovered GPG version
+type GPGVersion int
+
+// GPG version as discovered
+const (
+	GPG1x      GPGVersion = 1
+	GPG20x     GPGVersion = 2
+	GPG21xPlus GPGVersion = 3
+)
+
+var gpgVersionRegex = regexp.MustCompile(`\(GnuPG\) (\d)\.(\d)`)
+
+// GPGFinder implement search for gpg executables and returns version of discovered executables
+type GPGFinder interface {
+	FindGPG() (gpg string, version GPGVersion, err error)
+	FindGPGV() (gpgv string, version GPGVersion, err error)
+}
+
+type pathGPGFinder struct {
+	gpgNames     []string
+	gpgvNames    []string
+	errorMessage string
+
+	expectedVersionSubstring string
+}
+
+type iteratingGPGFinder struct {
+	finders      []GPGFinder
+	errorMessage string
+}
+
+// GPGDefaultFinder looks for GPG1 first, but falls back to GPG2 if GPG1 is not available
+func GPGDefaultFinder() GPGFinder {
+	return &iteratingGPGFinder{
+		finders:      []GPGFinder{GPG1Finder(), GPG2Finder()},
+		errorMessage: "Couldn't find a suitable gpg executable. Make sure gnupg is installed",
+	}
+}
+
+// GPG1Finder looks for GnuPG1.x only
+func GPG1Finder() GPGFinder {
+	return &pathGPGFinder{
+		gpgNames:                 []string{"gpg", "gpg1"},
+		gpgvNames:                []string{"gpgv", "gpgv1"},
+		expectedVersionSubstring: "(GnuPG) 1.",
+		errorMessage:             "Couldn't find a suitable gpg executable. Make sure gnupg1 is available as either gpg(v) or gpg(v)1 in $PATH",
+	}
+}
+
+// GPG2Finder looks for GnuPG2.x only
+func GPG2Finder() GPGFinder {
+	return &pathGPGFinder{
+		gpgNames:                 []string{"gpg", "gpg2"},
+		gpgvNames:                []string{"gpgv", "gpgv2"},
+		expectedVersionSubstring: "(GnuPG) 2.",
+		errorMessage:             "Couldn't find a suitable gpg executable. Make sure gnupg2 is available as either gpg(v) or gpg(v)2 in $PATH",
+	}
+}
+
+func (pgf *pathGPGFinder) FindGPG() (gpg string, version GPGVersion, err error) {
+	for _, cmd := range pgf.gpgNames {
+		var result bool
+		result, version = cliVersionCheck(cmd, pgf.expectedVersionSubstring)
+		if result {
+			gpg = cmd
+			break
+		}
+	}
+
+	if gpg == "" {
+		err = errors.New(pgf.errorMessage)
+	}
+
+	return
+}
+
+func (pgf *pathGPGFinder) FindGPGV() (gpgv string, version GPGVersion, err error) {
+	for _, cmd := range pgf.gpgvNames {
+		var result bool
+		result, version = cliVersionCheck(cmd, pgf.expectedVersionSubstring)
+		if result {
+			gpgv = cmd
+			break
+		}
+	}
+
+	if gpgv == "" {
+		err = errors.New(pgf.errorMessage)
+	}
+
+	return
+}
+
+func (it *iteratingGPGFinder) FindGPG() (gpg string, version GPGVersion, err error) {
+	for _, finder := range it.finders {
+		gpg, version, err = finder.FindGPG()
+		if err == nil {
+			return
+		}
+	}
+
+	err = errors.New(it.errorMessage)
+
+	return
+}
+
+func (it *iteratingGPGFinder) FindGPGV() (gpg string, version GPGVersion, err error) {
+	for _, finder := range it.finders {
+		gpg, version, err = finder.FindGPGV()
+		if err == nil {
+			return
+		}
+	}
+
+	err = errors.New(it.errorMessage)
+
+	return
+}
+
+func cliVersionCheck(cmd string, marker string) (result bool, version GPGVersion) {
+	output, err := exec.Command(cmd, "--version").CombinedOutput()
+	if err != nil {
+		return
+	}
+
+	strOutput := string(output)
+	result = strings.Contains(strOutput, marker)
+
+	version = GPG21xPlus
+	matches := gpgVersionRegex.FindStringSubmatch(strOutput)
+	if matches != nil {
+		if matches[1] == "1" {
+			version = GPG1x
+		} else if matches[1] == "2" && matches[2] == "0" {
+			version = GPG20x
+		}
+	}
+
+	return
+}