Display version string in footer

fixes: #528

Signed-off-by: Olivier Mehani <shtrom@ssji.net>
Overwrite version with a more informative git describe

Summarily tested to work both with and without a .git directory. In the
latter case, fallback to the hardcoded version in lib/version.js.

Version tags should probably be annotated so we don't need '--tags'.
Also, a v0.3.0-alpha tag is needed on 3a3fc500.

refs: #528

Signed-off-by: Olivier Mehani <shtrom@ssji.net>