RUBY-645 add equality + hash methods to wire protocol messages
RUBY-643 handle missing group in select target
RUBY-643 fix style issue and add test
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
Add rdoc to connect
Fix rdoc issues with attributes
minor: fixing attr_reader yard docstring format
trivial: getting rid of rubocop's complaints for the extra line here
refactor: making client context shared and updating scope specs