* restrict handling of callback return values for sockets:
  if the callback function returns False/None the callback is removed
  generic DONE
  gtk	  DONE
  qt	  DONE
  wx	  TBD (deprecated)
* add support for write notification
  generic DONE
  gtk	  DONE
  qt	  TBD
  wx	  TBD (deprecated)
* dispatcher support
  generic DONE
  gtk	  DONE
  qt	  TBD
  wx	  TBD (deprecated)

for 0.5.0
---------
* update README
* add simple thread handler
* check qt notifier