Fixed metaclass support in Python 3

[mmarzantowicz]

Use django.utils.six to allow metaclass support in Python 2 and Python 3.

[coveralls]

Coverage decreased (-0.08%) to 88.0% when pulling 6e8c98a on mmarzantowicz:feature/toolbar_py3_compat into f58060c on divio:develop.

[mmarzantowicz]

One thing needs to be clarified. I've chosen "older" with_metaclass implementation from django.utils.six (<=1.7) over that available in django CMS because it's sufficient. Ideally cms.utils.compat.metaclasses module would be replaced with six module already available in django. However, this must wait till django 1.7 is minimum supported version by django CMS. This is because version used currently by django CMS is available in django since about version 1.7.