renamed `Nelmio\Alice\ORM\Doctrine` => `Nelmio\Alice\Persister\Doctrine`

Same as #119 but on top of new 2.0 version.

As far as MongoDB fixtures can be runned on stock Alice without any changes of code (I have used this numerous times, and it looks like PHPCR fixtures can be applied as well) there is confusing namespace `Nelmio\Alice\ORM\Doctrine`. `ORM` part of namespace redundant there.

Or let's just rename `Nelmio\Alice\ORM\Doctrine` to something more common, like `Nelmio\Alice\Persister\Doctrine`