Display version number (from git describe when available) in footer.

This should fix #528 (at least, this does all I had in mind when opening the
issue).

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 3a3fc50.