Started documentation

This is the much needed skeleton to begin documenting Darcsforge much better. There is still a lot to write/fix, but it should serve as a starting point for some discussions as other parts of Darcsforge move closer to completion.

The documentation tool of choice is Sphinx, which is uber-cool.

Also included in this patch is a tiny amount of refactoring to darcs.py, primarily to fix some documentation comment bugs and to make the dependencies on Django and darcsforge.patches "softer".