Added python-cherrypy3, needed for the web interface

I installed tribler in a clean environment, and this module was missing
http://forum.tribler.org/viewtopic.php?f=6&t=5311&p=8838#p8838
 File "Tribler/Main/tribler.py", line 322, in __init__
from Tribler.Main.webUI.webUI import WebUI
File "/usr/share/tribler/Tribler/Main/webUI/webUI.py", line 1, in <module>
import cherrypy
ImportError: No module named cherrypy