Add API documentation

[njwilson]

We want pip2 to be usable as a library, so we need good API documentation.

An example project on PyPI has a section on Documenting Your Project Using Sphinx with useful information. It has sample docstrings using a "googley" style and a "sphinxy" style; I prefer the sphinxy style.

Once the docstrings are in place, we can use the Sphinx autodoc extension for our documentation.

Here's a real example from the Flask project: source and rendered documentation.

[njwilson]

Minimal API documentation is now available here: http://pip2.readthedocs.org/en/latest/dev/api/index.html