Adding replica set support for mongokit

ReplicaSetConnection added
Documentation, changelog updated
Using multiple inheritance for DRYness

Moves the monogkit specific funcitons to a new class. Clients now
inheirit from both the mongokit and pymongo client.

In particular, each client needs to run both __init__ from both.
Since mongokit comes first in the MRO order, register and __getattr__
will be from mongokit, with all other functions falling back to the
pymongo versions.
Merge pull request #123 from Ink/master

Adding ReplicaSetConnection support