Convert README.md to ReST and divide into several pages for sphinx

I've taken a stab at dividing up the README into several pages for sphinx documentation, and in the process I've converted the markdown to ReST.

You can see what the new README looks like as rendered on github here: https://github.com/jcrobak/luigi/tree/readme-md-rst-sphinx and I've uploaded the docs here: http://jcrobak-sandbox.s3.amazonaws.com/luigi-sphinx/index.html

The way I divided up is probably not perfect, but I think it makes the docs more approachable overall.