[WIP] Total efficiency calculation

I added two methods to the Efficiency class to compute the total efficiency (weighted by the bin contents of the total histogram). I also implemented the error computation (uncorelated accross bins).

Quentin
ah, missed this. rootpy tests should not depend on numpy if possible. If they do they should be skipped if numpy is not installed by raising SkipTest (see rootpy/tests/test_root2hdf5.py). Can you use ``FillRandom`` instead? That would also avoid the looping on the python side.