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