Mrtyler yo dawg debug in yo debug

Fall back to pdb if ipdb isn't available. In addition to being a lot nicer to the ipdb-deficient, it allows us to remove the hard Depends in the Debian package on ipdb (we use Suggests instead now).

I tried to write unit tests for the postmortem import behavior, but that (extracted) method uses nothing but print and import and I couldn't find a way to mock python keywords :s. I promise I tested by hand in an environment both with and without ipdb available.