Have tox install via setup.py develop

tox 1.6 was released, which means that we can now take advantage of
the feature we added to it - which is using setup.py develop to
install the code into the virtualenv. The logic was taken from
run_tests.sh - so the performance issues around using tox vs. using
install_venv should now be gone.

Additionally, override the tox pip install command to avoid using
the "--pre" option which is the default in tox. "--pre" means
"Include pre-release and development versions." By default, pip will
only install stable versions of software, and that is the behavior
we want.

Change-Id: Ida5e440d1bdb9f8e9031277ea53a02d2ef171438
Co-Authored-By: Monty Taylor <mordred@inaugust.com>

tox.ini

@@ -1,7 +1,11 @@
 [tox]
+minversion = 1.6
+skipsdist = True
 envlist = py26,py27,py33,pep8
 
 [testenv]
+usedevelop = True
+install_command = pip install -U {opts} {packages}
 setenv = VIRTUAL_ENV={envdir}
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt