Automatic versioning for aptly

New version format:

* for releases, `x.y.z` (follows tag without leading `v`)
* for nightly builds, `x.y.z+N+hash` (previous version, not the upcoming one)

This means that each nightly build `aptly` would report
correct version now.

Version is now complied into the aptly binary, system tests
automatically check for current version, no need to update them
anymore.

aptly/version.go

@@ -1,7 +1,7 @@
 package aptly
 
-// Version of aptly
-const Version = "0.9.8~dev"
+// Version of aptly (filled in at link time)
+var Version string
 
 // EnableDebug triggers some debugging features
 const EnableDebug = false