The django_evolution.evolve module is responsible for orchestrating

the entire evolution and migration process, and as such it's fairly

complex and large. In an effort to keep this maintainable, the module

has been split up into four sub-modules:

django_evolution.evolve.base
django_evolution.evolve.evolver
django_evolution.evolve.evolve_app_task
django_evolution.evolve.purge_app_task

No logic has changed. This is a straight-forward split of the module.

All unit tests pass.