take away migration, they are inconvinient for extensions

I think that putting the migrations in the pip package is not convenient for apps that extend Connection, Contact and Location models which are extendible. This would mean that whenever they add new extensions they will have South write into the pip package migrations.

I propose we advise the devs to instead use
SOUTH_MIGRATION_MODULES = { 'rapidsms': 'myapp'}

This seems a bit more decoupled, totally separating the rapidsms migrations from the rest of the code base.