3.0 behavior manipulation

With CakePHP 2.x it was possible to directly access the BehaviorCollection for a model in order to dynamically load, unload or alter an attached behavior. This was no longer possible with the new Table and BehaviorRegistry (With the exception of loading a behavior).

This pull requests adds the ability to remove behaviors and to retrieve the behavior instance that is attached to the Table.

It adds two methods to the Table class:
`$this->removeBehavior();` and `$this->getBehavior();`