WiP Django 1.6 support

While Django 1.6 is still unreleased, we can try to see what's going to happen.
Still a work in progress and only addresses python-level issues.
All dependencies should be 1.6 ready now.
~~Biggest issues at the moment are `django-reversion` (PR opened for 1.6 support)  and `django-mptt` support.
`djangocms-text-ckeditor` (opened PR for support) and `django-hvad` looks more easy to fix. 1.6 support fixed in `django-hvad`.~~
ATM code is django 1.4 compatible.
Django 1.6 added to build matrix to track the progress.