Changes handlebars dependency, adds tests, corrects asset include, bumps version to 1.0.2

* Changes `handlebars` gem dependency to [`handlebars_assets`](https://github.com/leshill/handlebars_assets)
* Adds includes for JavaScript library gems so they load via the asset pipeline
* Corrected handlebars use in `mozart-all` manifest to `handlebars.runtime`
* Updated gem description
* Corrected usage information in generator to match README
* Added tests for gem class constants :smile_cat: 
* Bumped gem version to 1.0.2