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

man/aptly.1

@@ -1,7 +1,7 @@
 .\" generated with Ronn/v0.7.3
 .\" http://github.com/rtomayko/ronn/tree/0.7.3
 .
-.TH "APTLY" "1" "November 2017" "" ""
+.TH "APTLY" "1" "September 2018" "" ""
 .
 .SH "NAME"
 \fBaptly\fR \- Debian repository management tool
@@ -150,7 +150,7 @@ don\(cqt verify remote mirrors with gpg(1), also can be disabled on per\-mirror
 .
 .TP
 \fBgpgProvider\fR
-implementation of PGP signing/validation \- \fBgpg\fR for external \fBgpg\fR utility or \fBinternal\fR to use Go internal implementation
+implementation of PGP signing/validation \- \fBgpg\fR for external \fBgpg\fR utility or \fBinternal\fR to use Go internal implementation; \fBgpg1\fR might be used to force use of GnuPG 1\.x, \fBgpg2\fR enables GnuPG 2\.x only; default is to use GnuPG 1\.x if available and GnuPG 2\.x otherwise
 .
 .TP
 \fBdownloadSourcePackages\fR
@@ -434,7 +434,7 @@ when processing dependencies, print detailed logs
 .
 .TP
 \-\fBgpg\-provider\fR=
-PGP implementation ("gpg" for external gpg or "internal" for Go internal implementation)
+PGP implementation ("gpg", "gpg1", "gpg2" for external gpg or "internal" for Go internal implementation)
 .
 .SH "CREATE NEW MIRROR"
 \fBaptly\fR \fBmirror\fR \fBcreate\fR \fIname\fR \fIarchive url\fR \fIdistribution\fR [\fIcomponent1\fR \|\.\|\.\|\.]
@@ -2038,5 +2038,17 @@ Matt Martyn (https://github\.com/MMartyn)
 .IP "\[ci]" 4
 Ludovico Cavedon (https://github\.com/cavedon)
 .
+.IP "\[ci]" 4
+Petr Jediny (https://github\.com/pjediny)
+.
+.IP "\[ci]" 4
+Maximilian Stein (https://github\.com/steinymity)
+.
+.IP "\[ci]" 4
+Strajan Sebastian (https://github\.com/strajansebastian)
+.
+.IP "\[ci]" 4
+Artem Smirnov (https://github\.com/urpylka)
+.
 .IP "" 0
 

man/aptly.1.ronn.tmpl

@@ -130,7 +130,9 @@ Options:
 
   * `gpgProvider`:
     implementation of PGP signing/validation - `gpg` for external `gpg` utility or
-    `internal` to use Go internal implementation
+    `internal` to use Go internal implementation; `gpg1` might be used to force use
+    of GnuPG 1.x, `gpg2` enables GnuPG 2.x only; default is to use GnuPG 1.x if
+    available and GnuPG 2.x otherwise
 
   * `downloadSourcePackages`:
     if enabled, all mirrors created would have flag set to download source packages;