added hotkeys to navigate in stream

Hey, my name is Steffen and this is my first pull request so please be nice ;-)
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

I'm a total newbie to this so I won't be surprised if there are things which need to be changed. Just let me know.