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.