Rails Assets management (and Bootstrap Formhelpers)

I played around a lot with www.rails-assets.org and bower-rails.

I had a lot of troubles to get [Bootstrap Formhelpers](http://bootstrapformhelpers.com/) running with both of them, but in the end I decided to go with rails-assets.org as it makes it more Rails-like to add assets and less configuration is required.

Still, Bootstrap Formhelpers doesn't work with it, so I had to add it manually myself in the app's assets. I have some tickets open (e.g. https://github.com/rails-assets/rails-assets/issues/148) which could solve this problem some time.

At the moment, only the `f.input :created_at, as: :date_picker` custom input exists. Others can be added similarly.