Add bowerDependencies() to Project

I have added `bowerDependencies()` to Project model. `dependencies()` was implemented for NPM before and for symmetry I added this. I also selfishly need this for [quaertym/ember-cli-dependency-checker](https://github.com/quaertym/ember-cli-dependency-checker) and thought it might be useful for other addon developers.
@brzpegasus yep.