Test py

This branch implements two new features:
1. an example (python/cutflow_2lep.py) showing how to
  access `SusyNt` objects and `SusyNtTools` functions from python
2. a generic script (python/plot_comparison.py) that takes two SusyNtuple
  inputs (directories, files, etc.) and makes simple plots comparing the same
  variables from the two inputs. So far, I used `2` to compare ntuples made
  from `NTUP_SUSY` and `NTUP_COMMON`

My plan is now to use these examples to write in python more accurate
tests to be run on travis-ci (using python.unittest, which is much easier
than running tests in c/c++).

Also, if anyone has time to expand `plot_comparison.py` with more
accurate comparisons, that would be really useful.