Proper Python package

I changed a little bit GWM layout so that it's now more like a True Python Package (tm).

**Important**
Completely removed deprecated stuff (it wasn't much, fortunately!)
Changed package name to `ganeti_webmgr` (from `ganeti_web`), so that it matches naming convention.

I also rst'd some documents and included (borrowed from @audreyr) `CONTRIBUTING.rst`.

This should be easy to merge.