added sync router
sync_schemas simplified, no longer needs to manually tell which apps need to be synced thanks to router
removed debug print
sync_schemas now raises exception if DATABASE_ROUTERS does not include 'tenant_schemas.routers.TenantSyncRouter'
updated docs regarding tenant_schemas.routers.TenantSyncRouter
fixed sync_schemas and migrate_schemas not taking arguments
Merge branch 'experimental-db-router'