Modify .travis.yml for matrix testing
Django 1.4.x is not usable with Python 3
Fix problem with testsuite modifying migrated model auth.Group

Django 1.7 comes with its own migrations framework and has bundled
migrations for all models. The test which tried to run `mptt.register()`
on auth.Group always failed because Django really does not want database
changes without migrations anymore.

We can easily substitute auth.Group with our own Group model: the test
suite only checks whether saving groups works (and saving would fail if
the database would not contain MPTT's internal fields for some reason.)
Fix all flake8 warnings but E128 and E501
Always be copy-pasting, always be failing
Remove FeinCMSModelAdmin

It has been deprecated for quite some time.

Fixes #240.
Fix a few doctest failures related to auto increment sequences

Now I feel dirty.
Remove a few unused imports
Take another stab at fixing the get_query_?set mess

Refs #316.
Why do we even try to work with get_query_?set when it would be sufficient to simply set _meta.ordering?

Refs #316.
Remove an unused import
Using TreeManager is not a sufficient reason for being patched

Refs #316.
Reindent the recursetree template snippet

Thanks to ticosax for the suggestion.
Python 2.6 does not know about the `flags` argument to re.sub
bump version to 0.7.a, as there's some backward incompatible changes in master now. Add to upgrade.rst