Add Gaze and subdirectory watching support

This adds [Gaze](https://github.com/shama/gaze) for better file watching support.  

This also extends the `watch` option to allow for recursive directory watching.  This is useful when we have one main asset which includes others, which may be located in a subdirectory. For example, having an a SnocketsAsset `application.js` require `./helpers/something.js`.

This is similar to #105, but that issue looks abandoned.