Add hjkl hotkeys for navigation

Vimers would be happy.
Also it's common hotkeys for web apps, if you doesn't believe me, believe [Google Reader](http://support.google.com/reader/bin/answer.py?hl=en&answer=69973), [Twitter](http://www.rewritetech.com/wp-content/uploads/2010/09/Picture-255-600x466.png) and [RememberTheMilk](http://www.rememberthemilk.com/help/answers/basics/keyboard.rtm).
Thanks.