ParGroup support for Supernova

This adds support for [ParGroups](http://doc.sccode.org/Classes/ParGroup.html).

To use Overtone with the Supernova synthesis server, the path to supernova to your `config.clj`:

```Clojure
   :sc-path "/usr/local/bin/supernova"
```

I'm very new to both Clojure and Overtone, so I'll be happy to refactor this or make additional changes to suit the style and/or design goals of the project.