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Then again, we want travisci and co to fail when sphinx build fails (maybe also when piprot says things are out of date?)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.
I've been using it for some years now, it's very handy. Basically it only tries to install new requirements if the requirements.txt file is newer than the requirements.txt.log file.