Fixes documentation building

 - Adds the ability to build docs using tox
 - Fixes autodoc generation

A Sphinx extension is introduced in the commit to facilitate building
the API documentation. This extension should be removed when
bug 1260495 is fixed.

Change-Id: Ibf5e5403cb7d3e67947c87b2828b64a56a11fc30
Merge "Fixes documentation building"