Cleaned up EmberApp.js

* added `inProduction` and `notInProduction` 'helper' methods
* removed `testing` and `hinting` properties
* removed unnecessary conditional around hinting
  * `testing` and `hinting` were always the same
* removed unnecessary file path munging
* pulled some logic out of the `javascript` method into `prepareJavascriptsForCompilation`
* simplified `injectENVJson` by pulling markup out of the logic and putting it in the template and refactoring