Added __len__ to AppenderQuery/AppenderMixin.

The main motivator for this is consistency in how relationships are handled. If I have backrefs that use len() and I then configure that backref to `lazy='dynamic'`, code breaks.

I realize using `len()` in the first place isn't the best idea in most situations, but its intent is to minimize differences in the behaviour of `lazy='dynamic'`. I hope it's not too heretical.