EvolveAppTask now has the capability to install models in the

database, creating all the necessary tables for the models and any

related models (such as for many-to-many relations).
It does this by first checking whether we're evolving an app for the

first time, or whether we're adding a model for an existing app for the

first time. These models, and the SQL needed to install them, are

gathered during the preparation stage. All new models across all apps

are then installed during EvolveAppTask.execute_tasks, before

performing any evolutions for any apps.
EvolveAppTask.execute can also take a create_models_now boolean

parameter, which will force installation of models immediately. This

will be needed for the initial installation of the Django Evolution

models in an upcoming change.

Unit tests pass for SQLite, MySQL, and Postgres on Django 1.6 and 1.11.