Add ember_cli_script_tags helper

Adds ember_cli_script_tags helper which is used to load the requisite ember files generated by EmberCLI.

The default behavior of the method add is as follows

- If app_name is given use that in the `javascript_include_tags`
- If app_name is NOT present attempt to look up the name of the first ember-cli app from `EmberCLI.configuration.app_list` (sensible default)
- if neither of the above steps are satisfied then render nothing (rather than break)

The complexity [here](https://github.com/rwz/ember-cli-rails/compare/add_view_helpers?expand=1#diff-27b789d467e5baac04888952066a8924R6) is because I didn't want to pollute the view helper namespace with extra methods (`private` was not obliged) and I didn't want to reach into `EmberCLI.configuration` unless app_name wasn't specified.