Add support for MongoReplicaSetClient

adds an optional replica_set argument to Session.connect, which uses
the MongoReplicaSetClient, if available, otherwise falls back to the
deprecated MongoClient(replicaSet=replica_set)
Merge pull request #136 from OpenLearningNet/master

Add support for MongoReplicaSetClient
Updated generated html docs
Automatic polymorphic queries

This change:
* makes superclasses know about all their subclasses, even multiple levels down
* Makes the queries for polymorphic documents include an $in query for the
  config_polymorphic field with the config_polymorphic_identity for all
  subclasses
* Adds fairly good tests for this feature

resolves #138