Django 1.7 compatibility

Fixes #12 

It changes PositiveIntegers to ForeignKeys to comply to Django 1.7 system checks requirements for GenericForeignKeys (renames couple of db fields as a side effect). And adds migrations for both Django 1.7+ and older versions (via South>=1.0) 

I'm not sure, should we change `related_object_site` too or not, so I left it as it was.