Make %pdb enable PuDB in IPython

This currently works only IPython 1.0.0, though it could probably be
backported.

It currently happens automatically when you import pudb.ipython (it works by
monkeypatching ip.debugger). For example, with this

In [1]: import pudb.ipython

In [2]: %pdb
Automatic pdb calling has been turned ON

In [3]: raise Exception

will enter PuDB.

The %pudb magic is completely unrelated at the moment.