Project Joining

Pivotal Tracker https://www.pivotaltracker.com/story/show/63372740
- implemented `act_as_follower`-gem. 
- created cukes and view specs. 
- updated views for `projects/show` and `devise/registrations/edit`
- added join and leave project buttons to `projects/show`
- added followers count to `projects/show`
- added links to followed projects in `devise/registrations/edit`