Fix to use bootstrap generator with a custom path with --ember_path option.

I ran into the problem in issue #74, running the bootstrap generator in a rails 3.x engine. The problem is that the generator requires the file app/assets/javascripts/application.js to exist, because the generator add to it. Rails 3.x engines use namespacing, so in an engine named custom, the application.js file will be found at app/assets/javascripts/custom/application.js. I tried to use the --ember_path option, however it didn't work because it was still looking for app/assets/javascripts/application.js. This simple fix uses the already existing ember_path variable to get to the application.js file. I also added test/dummy/app/assets/javascripts/custom/application.js, which was necessary keep the tests passing with this fix.