Add an easy way to set callback priorities for behaviors

Behaviors often use priorities, make it easy to define the priority
of callbacks a behavior provides when binding it to a table.
Improve method mapping in BehaviorRegistry.

* Omit methods on base class.
* Use ReflectionClass instead of global functions.
* Ignore methods starting with _. This omits methods like __construct
  and other PHP magic methods.
* Make method lookups case insensitve as this is how PHP works
  internally as well.