Added new command bb_schemamigration

- Add get_model_map function in model dispatcher
- Copied and adapted South schemamigration to replace base_model strings with mappings.
- Redid some migrations
Clean up
Add more mappings. New migrations
Add some comments
Merge remote-tracking branch 'upstream/feature/fund-loving-criminals' into feature/book-786-fix-migrations

Conflicts:
	bluebottle/utils/model_dispatcher.py
	bluebottle/utils/utils.py
Merge pull request #952 from onepercentclub/feature/book-786-fix-migrations

Added new command bb_schemamigration