Test against multiple Django versions

Adds support for tox to test locally.
Adds support for Travis.
Replace pyflakes with flake8

Had the dependencies the wrong way around.
Merge pull request #474

Conflicts:
	requirements.txt
Fix testing conflict

spec==0.10 uses an unstable version of nose that I don't really want to
couple Oscar to right now.  So I've kept spec at 0.9.7 (which requires
nose<1.2) and nose-progressive<0.4 so that we can use a compatiable nose
version.

Related to second point of #578
Merge branch 'master' of github.com:tangentlabs/django-oscar