Added 'handlebars_assets' gem as explicit dependency

This just includes `handlebars_assets` as part of the Mozart gem. In order to enable Mozart on Rails, `handlebars.runtime` needs to be explicitly added to the asset pipeline.

A future update should provide an asset pipeline stub which includes all the required dependencies by default. eg: `#= require mozart` should pull in something like the following:

```
#= require jquery
#= require underscore
#= require handlebars.runtime
#= require mozart.min
```