RUBY-686 URIParser support for auth mechanism and auth source
RUBY-686 MongoClient changes for auth refactor and light cleanup

* Using a Set instead of Array for MongoClient#auths
* Behavior correction: default DB is now 'admin' (same as all other drivers)
* Moved an undocumented internal method to private
* A little light style/format cleanup (if you touch it make it better!)
* Standardized on db_name over default_db in MongoClient
RUBY-686 test coverage for auth mechanism and auth source support

* additional tests to cover URIParser support for authSource and authMechanism
* updated existing tests for the related changes
RUBY-686 relocation and refactor of issue_authentication
RUBY-686 relocation and refactor of issue_logout
RUBY-686 legacy wrappers for issue_logout and issue_authentication
RUBY-686 updating pool and socket to use set comparison for auths
RUBY-667 user command helpers and update to auth tests
Add support and error for digestPassword option