Add SassAsset

I took a shot at adding a `SassAsset`.  I tried to keep it similar to the `LessAsset` coding style and functionality.  Same goes for the unit tests.

I had some trouble finding a good node sass compiler, the [node-sass](https://github.com/andrew/node-sass/issues) project seems to be unmaintained and has a problem with compressed output, so I wrote a little wrapper around the sass command line process.  It's a little slow, but seems to do the job.

Sorry about the package.json whitespace issues, all that should have changed in that file was adding a dependency on [node-sassy](https://github.com/jgable/node-sassy).