Modify setup to handle sphinx contrib requirements for docs.

ReadTheDocs.org relies on 'python setup.py install' to install any
non-standard dependencies in a virtualenv environment.  Both virtualenv
and readthedocs set environment variables when in use.  Thus we alter
the behavior when those are present.

- When in a virtualenv env, modify the install locations to root in
  our virtualenv space.
- When run by readthedocs, install from those requirements-doc.txt file
  instead of the default requirements.txt.