(SERVER-204) Subcommands for 'ruby' and 'irb'

Working implementations for ruby and irb to go along with the gem
subcommand. Acceptance tests need to be written, as well as docs.
(SERVER-204) Set compatibility version and improve load path order
Merge pull request #321 from nwolfe/svr204-ruby-subcommand

(SERVER-204) Subcommands for 'ruby' and 'irb'