Show subcommands in help output

![screen shot 2014-02-20 at 1 36 53 pm](https://f.cloud.github.com/assets/237985/2221848/1e76cdce-9a5e-11e3-8037-b599d6b9716b.png)

- [x] Print custom presentation of command when help switch passed
- [x] Allow subcommands to use `--help` and `--version` to show their respective subcommands
- [ ] Specs for `Mercenary::Presenter`

Fixes #15.