Add a callback mechanism to inject code globally into all tasks

In some cases, you need a bit of code to run before or after every task to handle global state initialization and/or cleanup. As a specific example, if you're using global SQLAlchemy sessions within the worker process, you need to close the session after every task to insure you don't reuse a stale session in future tasks. To provide an easy way for consumers to inject their own code into the worker process, I am proposing we add the ability to inject callbacks at key parts of the process.

The following callbacks are suggested as an initial set:

pre_execute
post_execute
pre_revert
pre_revert

To accomplish this, methods of those names should be added to the Task object, with a default implementation of nothing. Subclasses can define their own methods and those will be invoked as part of the task execution process.