Finishing Touches For PyPI

[sommersoft]

I was rolling through libraries that need releases. Upon reviewing this one, the only substantial change is adding Adafruit-Blinka to requirements.txt. However, I noticed that a couple things should probably be touched up before a new release:

• Add Adafruit-Blinka to setup.py->install_requires[]
• Add the PyPI install steps to the README

I should be able to get these easy bits done, but wanted to finish releases before then. Issue is mostly in case I forget to circle back around to this.