3.0 - Separate query->execute() & query->all()

Make the separation clear between `$query->execute()` which now *always* returns a statement object, and add `$query->all()` which triggers `Model.beforeFind`, and fetches result sets.  Having separate methods allows ORM\Query to honor the contract of the parent class, and also gives a shorter method for general use.

The documentation has already been updated in cakephp/docs#969