RUBY-652 Use allow instead of stub in Scope specs and Client shared context
Execute commands on a database.

This constructs a query message and sends it to the cluster to be
executed. Currently mocked until read preference and cluster are
complete.
RUBY-592 Initial Cursor implementation and specs
RUBY-636 RUBY-592 Adding enumeration with Cursor on Scope
RUBY-592 Updates to Cursor and specs after code review
RUBY-652 RUBY-653 updating specs to use allow instead of stub
RUBY-656 Cursor documentation