Fix circular import errors in cms.compat

For some reason, certain imports in cms.compat caused circular import errors when deploying using uWSGI or running ./manage.py cms.check (though runserver worked OK). Fixed by hiding the imports in function calls so they don't occur when cms.compat is imported.

Example error traceback: https://gist.github.com/koirikivi/9090176
The scaffolding I used for testing: https://github.com/koirikivi/django-cms-uwsgi-test

Unfortunately, I don't have tests reproducing the behaviour integrated to the cms test suite, but it's reproducible by creating a simple project and running cms check in it.

I ran the cms test suite on a headless virtual machine, which I presume caused some (8) tests to fail, but they weren't of related modules and weren't affected by my commit. I also tested the change manually using python2.6 and Django 1.6, both with a custom user model and without one.

I'm sending this PR to you since I think that you're the person in charge of this feature. Tell me if you want me to send it to divio/django-cms instead.