py.test.set_trace() doesn't work without -s option

Hey,

I just tried out your plugin and it works pretty sweet, except for set_trace function seemed to fail because stdin was being overridden...  I initially used a hack to replace the stdoi streams but this didn't seem to work reliably, so here is a patch that uses the py.test capture API to revert the IO streams before starting the debugger.  I basically copied the code from the py.test pdb module and i'm not sure what the license requirements are for the MIT style licences.

I have tested this with the following configuration.

pytest-ipdb 6580b68
python 2.7.3
py.test 2.3.4
ipython 0.13.2

Cheers,
Russell