restore auto debug behavior

[minrk]

This restores the auto debug behavior of the '%pdb' magic.

I'm not 100% sure this is the right way to do it, since the previous code looked weird, and I want to be sure I'm not doing something wrong.

It definitely works for me, though.

This will close #296

[takluyver]

Works for me.

[fperez]

Thanks, Min! I edited the commit so it would auto close the bug and rebased to avoid the merge "handle" since it was a single commit. Pushed.

There may be a cleaner fix for this later on, I suspect we should spend some time disentangling that code better (the old stuff had very tightly coupled debugger/traceback/interactive calls). But for now this closes an important regression, so let's go with this fix. Thanks!