Easy--go and get a bolt extracter set. If you have a Harbor Freight nearby, they have a set for around $10. Home Depot will be higher, but still the worth the money. Basically, you drill a small hole into what's left of the broken bolt and then insert a reverse threaded bit that bites into and backs out your broken bolt. (The set will have various sizes to use.) Might seem like a disaster to you now, but it's really no problem with the right tool.