Rails engine integration

This pull request makes it easier to run the bootstrap generator within a rails engine. It will detect that the generator was run from within a rails engine by checking that EMBER_PATH is defined. If it is, it uses EMBER_PATH to get the name of the engine. This gives us the name of the folder in app/assets/javascripts that contains the application.js file, and will be the home of the bootstrapped ember app. This is tested with a dummy_engine added to the project under the test directory. A new test helper is used to load the dummy engine.