API documentation

[faassen]

We need to generate API documentation in our sphinx documentation. For this we need to add the appropriate docstrings to the API too.

[faassen]

This is becoming more pressing; would be the best way to document the renderers and various other behaviors.

[faassen]

It'd amount to creating an api.rst with the right directives and including it in the toc. Here's Morepath's example:

https://raw.githubusercontent.com/morepath/morepath/master/doc/api.rst

We'd need to do this for everything in __init__.py at least (also see #39).