RUBY-686 some additional authentication prep and cleanup

* standardizing some terminology in client/db (connection -> client)
* removed yardoc extension and update yardopts
* removed @core tag references (mongodb.org docs don't support this anymore)
* added some references to thread back so that if the order of things in the loadpath change references to Mutex/ConditionVariable will still work.
* moved authentication related functionality to a new auth module
Merge branch '1.x-stable' into 1.x-stable-ruby-683