Adding a Makefile

Partly stolen from #317. (Thanks @kevinburke! Thoughts?)

Was considering doing separate `dev-requirements.txt` (sphinx, piprot, etc) and `test-requirements.txt` (nose, tox, etc) but... so many requirements. :(

Then again, we want travisci and co to fail when sphinx build fails (maybe also when piprot says things are out of date?) #414, so merging these things might be a Good Thing.

The plan is to add my release routine in here, too.
Oftentimes I run scripts like this having forgotten to increment the version number.