allow keyboard interrupt to break out of ipdb

@takluyver @minrk and I discussed this issue in person.

It's currently problematic that there is no way, short of restarting a kernel,
to get out of an ipdb session if you've deleted the output of the cell in the
notebook which started it (by e.g. re-executing that cell).

This patch makes Ctrl-C behave the same as Ctrl-D inside of ipdb - it exits
the ipdb session. This also makes it possible to stop ipdb sessions from
another client.

I polled @katyhuff, @scopatz, and some other pyne/pyne hackers and they were
surprised to hear that this was not the behavior already.