Add git tag / SHA1 as a build string

Every build from commit 20271c6 (Apr 10 2013) is versioned as 1.6.0.
Add a build version after the version number
Digital Speech Decoder 1.6.0 (build:v1.6.0-20-g16110bd)

The build string uses git describe to list the tag closest to head,
and the number of commits past that tag, the a litteral g and ending
with the current SHA1
Merge pull request #15 from dreinhold/add_git_sha_version

Add git tag / SHA1 as a build string