Add file that uses specific gemset

In order to manage our project dependencies (mainly with Susy 2.0), we can start using Gemsets.

This file will assume that you have created a gemset named 'default', which upon entering this directory, either RVM or Rbenv will switch you into that gemset, and take you out of it when you leave.

In the future if you have project specific gemsets, you can switch the name of the gemset in this file and use your custom gemset. 

In order to use the Gruntfile in this repo, you'll need to create a default gemset and upgrade some gems:

In RVM:

```bash
rvm gemset create default
rvm gemset use default
gem install compass -v 1.0.0.alpha.19
gem install susy -v 2.0
```