Sphinx documentation

Here is a conversion of the existing material, with cleanups.

There are a few missing bits, I'll ask Tommi about them later.

The examples stay the same, I might push them out of the /doc directory later.

Quick and dirty hack for converting the DIA files to PNGs via the Makefile. It would be nicer to use Phix (an Sphinx extension) but development stopped in 2011 and there is currently no PyPI package for it.

This should close: https://github.com/twisted/ldaptor/issues/14