(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.
Even though it appears 1.9 is the default, might be good to set the version of Ruby explicitly here - like we do for the `ScriptingContainer` setup in Puppet Server production code:

```
(.setCompatVerison (CompatVersion/RUBY1_9))
```