expose IPython's pdb instance

[Paul Ivanov]

It would be useful to expose the particular instance of pdb in a clean manner.

Use case: I improved the Vim-IPython integration to set breakpoints from inside Vim (lp:~pivanov/ipython/devel), but in order to get access to pdb, I had to resort to using __IP.InteractiveTB.pdb, which only gets set after a debug call is made. Moreover, from my reading of the code this isn't even used if pydb is installed, though the appropriate pydb object would be able to handle all of the same calls (set_break,clear_all_breaks,run('execfile()')).

[Fernando Perez (fdo.perez)]

Related to this, we should also make it trivial to do the equivalent of

import pdb; pdb.set_trace()

but with IPython's pdb. Something like

from IPython import ipdb; ipdb.set_trace()

or perhaps with a different spelling (I've never liked 'set_trace', I think it's a rather obscure naming).