Basic implementation of Mongo::Client:

This also includes skeleton Collection, Database, and Cluster, since I
did not want to mock any_instance.new.
Ensure specific error raised when no db selected
Auto-use database when provided as an option.
Implementing Mongo::Client#with.
Implement equality checks in client
Adding client inspection
Initial database implementation
Make todo note on command, pass db to collection
Fix 1.8 hash syntax
Change test syntax
Return empty hash for options if none provided.
Adding in client connect from uri