Sphinx docs

Thought it might be nice to provide a little more hand-holding than "go read the tests".

I like Sphinx, but if you don't (e.g. prefer simplicity of most everything in top level module docstring) then feel free to close.

I've set up the code examples so they can be run with 'make doctest' and have checked that they run on different versions. The example code might need tweaks. This could be prep for putting docs on ReadTheDocs, Github pages or whatever.