`isEntityUsingTrait` makes more sense, doesn't it?

Anyway, isn't the goal of this PR to abstract wether this listener applies on Document or Entity?
In this case, this method could be called `isObjectUsingTrait`

Last thing, as I believe these 3 methods will be usefull in many other places, you might want to move it to its own class and inject it in the listener (something like `Knp\DoctrineBehaviors\Reflection\ClassAnalyzer`)