Feature/activity feed

This pull request adds an activity feed using [public_activity](https://github.com/pokonski/public_activity) to Latter. At the moment, it tracks Game creation, and game completion, but we have the ability to add additional activity tracking very, very easily.

The presentation of activities at the moment is very simplistic - there are improvements that can be made over time, but this will become more obvious when activities begin tracking on the live application.

The presentation at the moment is:

![screen shot 2014-01-23 at 12 11 58 pm](https://f.cloud.github.com/assets/292020/1980087/a3d3e408-83ba-11e3-9c63-1228ed60d03f.png)

The original `Activity` model has been removed in favour of using a gem for this, as the current model was not quite as flexible. The `Gemfile` has also been commented to explain why each gem is necessary, with a tidy up of dependencies as well.