+ 1 on having it done by someone with the right tool. If the hanger does snap off you may be able to replace the derailleur with one which has a claw-type mount. Something like the "Tourney Magarange" one on this page should work at moderate cost:
http://sheldonbrown.com/harris/derailers-rear.html
EDIT: Here's another one:
http://www.amazon.com/Shimano-Tourne.../dp/B003ZMH69S
Last edited by dsbrantjr; 02-23-13 at 11:08 AM.