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)




This test and the test before assumes that the `build_stubbed` instance will return a valid model, if it was invalid in the first place, these two tests would mean nothing :wink:. Therefore it is good practice to test that your factory is valid in the first place, with a test that goes something like: `it 'has a valid Factory' …`

On a separate note, since we are introducing a new model, I would like to suggest using the amazing [shoulda-matchers](https://github.com/thoughtbot/shoulda-matchers) gem for testing validations like these. It is far more expressive and very concise!