Support Django 1.7 migrations

@stephenmcd this is mostly just renaming migrations > south_migrations and adding a migrations folder, following along from this blog post: http://treyhunner.com/2014/03/migrating-to-django-1-dot-7/

I made a change to mezzanine.pages.fields as Django 1.7 migrations do not support lambdas in fields per this ticket https://code.djangoproject.com/ticket/22892

If you like it, I believe this closes #987 

I wasn't sure how to write tests for this, but it worked for me with 
- Django 1.6 + South 0.8.4
- Django 1.6 + South 1.0
- Django 1.7