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.
`require` will throw an exception if the file doesn't exist. Would be best to check for it's existence first before requiring it