Awesome keyboard shortcuts for GitLab

Tired of using your mouse for GitLab? :zzz: Here are keyboard shortcuts for GitLab. :metal:

An image says more than 1000 words. This PR improves GitLab with the following shortcuts:

![screenshot 2014-08-31 23 55 01](https://cloud.githubusercontent.com/assets/278301/4103361/82f7ed0e-3159-11e4-9f0d-1bda7b078838.png)

#### Implementation

I use [mousetrap](http://craig.is/killing/mice) for the key binding management. I first wrote a custom library, but it's not that easy to write a reliable library for that which works for different browsers and different languages, so don't reinvent the wheel!

For different pages, different shortcut handlers are used (derived from base shortcut handler, to support *help* and *search*). Additional, custom shortcut handlers define their visible help sections.