Migrations for Django 1.7.

Move South migrations to south_migrations, and add a warning to update
your settings if you try and run migrate with <1.7.

Thanks to http://treyhunner.com/2014/03/migrating-to-django-1-dot-7/ for
the idea.
lib2to3.fixes.fix_except
lib2to3.fixes.fix_idioms
lib2to3.fixes.fix_import
lib2to3.fixes.fix_ws_comma
lib2to3.fixes.fix_xrange
libmodernize.fixes.fix_dict
filter
Wrap map() in list/tuple, it's an iterator in Py3.
libmodernize.fixes.fix_metaclass
libmodernize.fixes.fix_next
libmodernize.fixes.fix_raise
Update imports using six.moves.
raw_input
Fix urllib
Fix print handling.
Fix unicode handling.
Python 3 string related fixes.
Update model string methods for Python 3.
Test Python 3 on Travis.