Add API documentation

Add docstrings and sphinx structure to expose them through the developer
docs for the library.

Also add a 'docs' target in tox.ini to make building the documentation
locally easier.

Remove the nearly useless usage.rst file.

Turn on the pbr flag to treat doc build warnings as errors.

Closes-bug: #1329952

Change-Id: I274a4144edf4efe9ed786e220e40003fda6f99e0
Merge "Add API documentation"