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

* Changes handlebar gem dependency to handlebars_assets
* Adds includes for JavaScript library gems so they load via the asset pipeline
* Corrected mozart-all manifest to use handlebars.runtime
* Updated gem description
* Corrected usage information in generator to match README
* Added tests for gem class constants
* Bumped gem version
Merge pull request #5 from sgerrand/switch-handlebars-dependency

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