New library to solve convex problem

 Changes to be committed:
	new file:   sparse_method.py
          This library contains a single function which uses cvxpy in order to solve
          a convex optimization problem which reweights marginal & joint entropies
          in order to yield a sparse normalized mutual information matrix. I think
          this will be very useful in the future.
          sparse_method depends on cvxpy (http://cvxpy.readthedocs.org/en/latest/index.html)