[WIP] Convert docs to reST

This is a proof of concept for our docs in reST format and fixes #71. I'm now convinced by the two advantages mentioned in #71 and think we need to do this conversion before 1.0. Comparing `varnish-configuration.rst` with our current `varnish-configuration.md` you'll see that the reST version is much leaner because we can just include the VCLs (even only parts of them, where necessary) we already have for our functional tests. This will lead to less mismatches between our docs and test setup, and also ease maintainability of those two. What do you think?