Py3 rebased

On the current Python3 branch, when running 'tox-epy27', I get:

FAILED (failures=6)
FAILED (errors=2, failures=8)

After rebasing the branch on top of master:

FAILED (failures=6)


It would be best to have a working test suite on Python2 before trying to port HTTPretty to Python3: this will avoir introducing regressions.