Add `current_user` options to navbar

[Pivotal Tracker story](https://www.pivotaltracker.com/story/show/64444298)

- Created `navbar_userinfo.feature` with scenatios
- Modified `spec/view/layouts/application.html.erb_spec.rb` to test for new layout
- Added a method in`application_helper.rb` to gather `current_user` information
- Added `current_user_details`-method to `_navbar.html.erb`-partial
- Added `current_user`'s small gravatar to  `_navbar.html.erb`-partial
- Created a pulldown menu for  `current_user` and mover 'My account' and 'Log out' links to that menu.

- Precompiled assets with `RAILS_ENV=production bundle exec rake assets:precompile`