Handle keystrokes in the web interface remote (up, down, left, right,… 

This pull request adds the possibility to press common buttons from the web interface remote using the keyboard:

 * left
 * right
 * up
 * down
 * play/pause (space bar)
 * OK (enter)
 * home (escape)
 * back (back space)
 * volume up (+)
 * volume down (-)

I Hope it will help

Fabien