Allow pluggable manager for TaggableManager.
Wrong south setting in the docs

Should be `SOUTH_MIGRATION_MODULES`, see:
http://south.readthedocs.org/en/latest/settings.html#south-migration-modules
fix contenttypes.generic warning in django 1.7
manager default to _TaggableManager in kwarg, swap class placement for import.
Tests and test model for custom manager.
Docs update
Merge pull request #216 from collinanderson/develop

fix contenttypes.generic warning in django 1.7
Merge pull request #214 from MeirKriheli/develop

Wrong south setting in the docs
Merge pull request #213 from nicholasserra/nicholasserra-pluggable-manager

Allow pluggable manager for TaggableManager.