(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.
If users are running `puppetserver irb` (or `lein irb`) then I think the arguments should go to `irb` and not `jruby`. If they want to access jruby then they should use `puppetserver ruby`. 

So I don't think it's necessary to make the code flexible enough to allow them to access jruby from any of the `irb`, `gem`, etc commands.