Add new website/documentation (using Sphinx)

This is a rebased version of [docs2.0](https://github.com/zoni/err/tree/docs2.0), with all commits squashed into one to keep history clean.

It's finally at a point where I'm pretty happy with it :) I tried to put a Google+ button in the sidebar as well, but couldn't get to work play nicely, so I dropped it entirely. I did however add links to the Google+ page in relevant sections, though.

Instructions on how to build this locally can be [found here](https://github.com/zoni/err/blob/sphinx/docs/contributing.rst#contributing-documentation--making-changes-to-the-website).