Rails engine integration

Integrate rails engine support into the bootstrap generator. If the generator is run from a rails engine directory it will detect that it is from an engine and extract the name and determine the proper javascript dir to place the generated files into. This is necessary for rails engines because an engine's javascript dir has an extra folder level for example `app/assets/javascripts/my_engine/`. Previously the generator would fail because it would not be able to find `app/assets/javascripts/application.js`. It uses the constants ENGINE_PATH and ENGINE_ROOT to detect a rails engine and get it's name.