[GaussianInference] can now init `alpha`, `m`, `W` from a Gaussian mixture
clean up

o [hierarchical.py] replace "np.empty(self.nin) + np.inf" by
                    "np.zeros(self.nin) + np.inf" because "np.empty"
                    may produce arrays with nans (happend on my
                    machine) and nan + inf = nan but all comparisons
                    with nan involved evaluate to False
o [plot]            moved imports from 'matplotlib' inside the function
                    "plot_mixture" --> raise 'ImportError("can't import
                    matplotlib")' on call
o [setup.py]        added directive 'extras_require' --> need 'nose' for
                    testing and 'matplotlib' for plotting