Use requirements.txt in setup.py

[WyattBlue]

This puts all dependencies in one place without repeating ourselves.
I also changed "Development Status" to stable since we care about backwards compatibly and since this program seems stable 😄

[slhck]

Sure, that's reasonable! As for the requirements vs. setup.py issue, I believe the best way would be to have setup.py specify abstract dependencies (i.e. without version pins), and requirements.txt to include the specific version pins we need. At least that's what I see recommended in various places. I pushed changes to master directly!