User status

PT feature: https://www.pivotaltracker.com/story/show/78088070

As site administrator
In order to simplify interactions between users
I would like them to be able to set current availability status on their user profile.

- added `online?` method to user model to see if user is online
- added `user_activity` method to application controller to touch user
- added migration to create a new status table with an associated model
- added UI to display if user is online (green dot)
- added UI to set user status on user profile page
- added UI to display status user profile and user index pages
- created feature scenarios and rspec tests

![screen shot 2014-10-03 at 15 48 39](https://cloud.githubusercontent.com/assets/1185760/4507438/869594c6-4b0c-11e4-82c1-29d0d4212c52.png)
![screen shot 2014-10-03 at 15 50 05](https://cloud.githubusercontent.com/assets/1185760/4507450/a09a9db2-4b0c-11e4-8d5e-5b3867b9992a.png)




I would suggest using `let` over setting instance variables (`@user`), could we update these tests to use `let` exclusively?