Simple Form Enhancements (Bootstrap3 and Datepicker)

Bootstrap 3 is working now, but we have to use the master branch as a new gem version of simple_form isn't released yet.

Also added [bootstrap-datepicker-rails](https://github.com/Nerian/bootstrap-datepicker-rails) gem and an input which can be used like `f.input :birthday, as: :date_picker`.