Persistent Bags / Multisets

Here's an implementation of a persistent bag/multiset, using a pmap to count elements.

This is pretty similar to collections.Counter in the Python stdlib, but less leaky :)

If you think it's not appropriate for pyrsistent, that's fine, I can release it as a separate library.

Since the PBag class is private (as are PMap, PVector etc) I decided to prefix it with an underscore.

Not sure six is needed here since you're operating on a pmap that does not change behaviour with python version.

I haven't really decided on how to handle the interface differences for access of keys, values and items between python 2 and 3 dicts but for now I think that sticking with the python 2 way will do.