Fix requirement handling in tox

Move all test dependencies to test-requirements.txt and update
tox.ini to use that and the main requirements file to build the
virtualenv for testing. Remove redundant references to the different
requirements lists in other tox environment stanzas.

Change-Id: I1ff094778369ebfed567ebbb433ef487066f042a
Fix incorrect image reference in documentation

Link to the correct image for the "API Enforcement" section of the PyCon
2013 essay.

Change-Id: I844e6dcd2dccd9367cc5c49f13b9abfc01b69f4b
Updated from global requirements

Change-Id: I2ae513d7d37a709b9c8964e99dc1db591e754d7e
Merge "Updated from global requirements"