Add autogenerated documentation with Sphinx

[cthoyt]

References #13 (but doesn't close it). After checking out this PR, you can learn a bit about how to use Sphinx to automatically incorporate your documentation into the Sphinx build. To build the docs, you'll start in the root of the repository and do the following in the bash:

pip install sphinx sphinx_rtd_theme # only have to do this once
cd docs
make html
open build/html/index.html

Rerun the make html command any time you make a change