sql_create(), a utility function for generating the SQL for creating

an app's models, has been renamed to sql_create_app(), and its

model-creating functionality split out into sql_create_models(). This

allows callers to generate suitable SQL for creating only a subset of

models for an app, or several models for several apps.
db_router_allows_schema_upgrade() is a new wrapper around

db_router_allows_migrate() and db_router_allows_syncdb(),

simplifying the work required by a caller.
db_get_installable_models_for_app() returns all models that can be

installed in the database (using sql_create_models() or another

operation). It takes care of returning models suitable for the preferred

creation mechanism used on the running version of Django, and handles

checking the router using the new db_router_allows_schema_upgrade().

Unit tests pass on Django 1.6 and 1.11.