Improve test suite and add contribution guidelines

- Add project to the [Sache](http://www.sache.in/ "Sache: Find Sass and Compass Extensions for your next project") registry
- Add contribution guidelines
- Write tests for #5 /cc @HugoGiraudel

![ ](http://gifs.joelglovier.com/contempt/stevie-mcquinn.gif)