(SERVER-253) Documentation for subcommands

Considering that most of these commands are standard Ruby ones I didn't feel the need to go into much detail. This ended up being mostly about showing examples of each one and calling out the `--` when running from source.

Happy to go change any/all of this; I just wanted to get something up before the break!
For `irb` and `ruby` we don't seem to unset `GEM_PATH`.  Should this also be reflected in the documentation?

```sh
$ GEM_PATH=foo lein ruby -c ~/.puppet-server/puppet-server.conf -- -e 'puts ENV["GEM_PATH"]'
foo
```