Automatized computation of p-value = f(m_H)

Hi Noel,

Could you pull this from me ?

I added a new module sig_severalmasspoints to compute and store in a pickle the significance for different mass points.
The module has a --force-pickle option that force it to re-compute the p-values and rewrite the pickle.
As we discussed, I'll try look up at the multiprocessing.proc module to speed up the computation.

Other minor changes:
--> I added a new category list in mva/categories/__init__.py for the 1J category
--> I added the numJet variable in the shape plotbook. Since we usually state that one of the good thing of the embedding/simulation is that it models better high multiplicity events.

Cheers,
Quentin