Extensible model choice list

Hi guys,

It should be possible to extends from `Sonata\AdminBundle\Form\ChoiceList\ModelChoiceList`. At the moment it's not possible, essentially due to the private `$entities` property that is modified in the `load` method.

Let's say I want to change the way the choices are built  (group them by type, or filter them, etc...) and I want  to preserve the current behaviour i.e. the way entity collection is populated.

The only commit mandatory here is to change `$entities` property visibility from private to protected.
The others are just some refactoring for consistency's sake : 
* php doc fix
* add a missing getter
* use already defined getters in the class

Cheers.