added hotkeys to navigate in stream

This pull request will add the following hotkeys to diaspora:
- "J": go to next post (and highlight it)
- "K": go to previous post (and highlight it)
- "C": jump into the comment-textarea of the highlighted post
- "L": like the highlighted post

'J' and 'K' will work on keydown, 'C' and 'L' on keyup to make sure they are triggered just one time.