Make Oscar work with Django 1.5

*This is work in progress branch for making Oscar work with Django 1.5*

Tasks
- [x] Fix validation issue
- [x] Adjust issues with `verify_exists` for the `EnhancedURLValidator`
- [x] Fix webtest issue with missing `streaming` attribute (see https://github.com/kmike/django-webtest/pull/3)
- [x] Solve issue with models not validating
- [x] Fix validation issue with logging in sandbox
- [x] Set up Tox and Travis so continuous integration tests against both Django 1.4 and 1.5
- [x] Fix issue with missing content-type on syncdb - see https://groups.google.com/forum/?fromgroups=#!searchin/django-oscar/django$201.5:$20cannot$20syndb/django-oscar/JJwlin50RYM/RIRDyPEhJ7AJ
- [x] Resolve django-treebeard issue
- [x] Support Django 1.5 custom User model support