Add methods and members to ManyToManyField

My code uses some methods and members on ManyToMany fields that pylint-django doesn't represent in the transformed class. This pull request adds those and more.

I most care about add(), clear() and through existing.

However, I went into Django source code at https://github.com/django/django/blob/1.7/django/db/models/fields/related.py and extracted all the public methods and members from the dynamically generated ManyRelatedManager, only omitting those members/methods which overlapped with those in QuerySet which is already inherited from.

This solves my problem, but does have the potential of being problematic for these reasons:
* Since the fields are returning None and no method bodies exist, the type inference in pylint may end up incorrect. I think this precedent has been set by the other transformations.
* get_queryset and get_prefetch_queryset have different name get_query_set/get_prefetch_query_set in the Django 1.5 series and earlier. For now, I've just assumed 1.6+ compatibility.
* add() and remove() may not exist for ManyToMany fields "if the ManyToMany relation has an intermediary model" in Django 1.6 series and below. I think this is okay because in 1.7 the methods always exist but raise exceptions if called in those situations; however 1.6 and earlier code may end up with false negatives if analyzing code that attempts to call add() and remove() where this situation applies.