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.

I'm a fan of examples in the doc strings of the public API of libraries. Even though these functions are very straight forward I still think that a basic example would make it even easier to grasp the functionality. Nothing complex or corner caseish, I don't want to have a large overlap between the doc and the unit tests. There's a doctest job running to make sure the examples are correct (except for the README which I just noticed is now incorrect :-)).