GooFit API docs

We should create GooFit API docs and put them on the web.

I'm trying out [doxygen](http://www.stack.nl/~dimitri/doxygen/) and [sphinx](http://sphinx-doc.org) now. Probably we can use [gh-pages](http://pages.github.com) or [readthedocs](https://readthedocs.org) to host them.

readthedocs has the advantage would be that the docs are automatically re-build for each commit, and there can be multiple versions (0.1, 0.2, ..., master). It only hosts sphinx docs though, which is mainly for Python docs, but apparently can also do C / C++ docs ... let's see.