Initial sketch of multi-VCS support.
Reformat some docstrings.
Update docs.
More documentation updates.

Don't reference git so much.
Add decrement_dev_version.

I hope this is as useful as I think it is.
Fix hg detection for decrement_dev_version.
Update the docs even more!

Remove still more references to git.
__sha__ -> __revision__

It's more general, because bzr and crap.
Add a bunch of tests; fix code in the process.