WiP Django 1.6 support

While still unrelease, it reached beta1, so we can try to see wha't happening.
Sill a work in progress.
Biggest issues at the moment are `django-reversion` and `django-mptt` support.
`djangocms-text-ckeditor` are easily fixed (will open issue against them)
ATM code is django 1.4 compatible.
Django 1.6 added to build matrix to track the progress.