Implement common set interface methods in shared sets.

This branch adds common set interface methods to the `SharedSet` class. The method descriptions from the set interface can be found [here](http://docs.python.org/2/library/sets.html). It implements each of the methods for comparison between sets and updating a `SharedSet`. Each comparison or update type method supports both the regular Python `set` and `SharedSet` arguments. Also, this implements the "magic" methods associated with each of the set interface methods. I believe just like the `dict` methods, these additions should make shared sets much easier to integrate into existing code.