Expand on docs around event priorities.

Add some detail around how priorities are handled.
Move controller resolution into a separate dispatcher filter.

Having a separate filter that handles controller resolution provides
a few nice things:

* It makes it easy for applications to extend/replace the controller
  conventions.
* It makes dispatcher super simple internally.
* It makes CakePHP less magic.

No unit tests were added as the existing Dispatcher test cases cover all
the functionality in the ControllerFactoryFilter.
Remove constructor that does nothing other than bad things.

Spooky side-effects at a distance on object construction is a no-no.