Add MongoClient class, to supersede existing Mongo class, with a default WriteConcern.ACKNOWLEDGED

Add a new class MongoClient with a default write concern of WriteConcern.ACKNOWLEDGED. Users of the Java driver should construct MongoClient instances in preference to Mongo instances.

To ease migration, MongoClient will extend the existing Mongo class. In order to ensure that the default write concern is always ACKNOWLEDGED, new companion classes, MongoClientOptions and MongoClientURI, will also be added.

MongoClientOptions differs from MongoOptions in that all fields will be final (and private), making it immutable. Instances of MongoClientOptions will be constructed with a static inner Builder class. MongoClientOptions will have a default write concern of WriteConcern.ACKNOWLEDGED.