RUBY-791: SCRAM-SHA-1 Authentication

This is the SCRAM-SHA-1 implementation for the 2.0 driver.

cc/ @behackett 
This shouldn't be its own attribute, it will be available through :auth_mech_properties as of this commit
https://github.com/mongodb/mongo-ruby-driver/commit/a4e709798c0d4333ade4066595b3ed5dcadff44c