Evolver now groups tasks by their type, and works with them in batches

based on the type. That means all tasks of a type will be

prepared/executed at once.
It's now up to the task's class to handle the preparation and execution

of the tasks. This allows them to compute state before or after, and to

alter the state passed to each task.
When executing tasks, each batch of tasks of a given type are, by

default, executed in a transaction specific to the batch. Subclasses can

change the logic as needed for transaction handling.

Unit tests pass on Django 1.7 and 1.11.