ADDED: matplotlib_utilities module

- Moved functions that enhance matplotlib (colormaps, normalizations
etc.) to new module. Module is imported with try-except statement since
functions require matplotlib which may or may not be present.

- Updated wigner_cmap to allow for custom colors that can be defined in
any format acceptable to the Matplotib.colors.ColorConverter.
That is a possibility.
> On Nov 3, 2014, at 15:20, Robert Johansson <notifications@github.com> wrote:
> 
> In qutip/__init__.py:
> 
> > @@ -240,3 +240,9 @@
> >  from qutip.utilities import *
> >  from qutip.fileio import *
> >  from qutip.about import *
> > +# try to load utilities dependent on Matplotlib
> > +try:
> How about just making a local dummy class called Normalize if matplotlib is not available then, just to make the definition of the class pass? i think if is nicer if the the presence of absence of optional dependencies does not change what symbols qutip provide, so that at least the functions and their docstrings etc are available. but in this case maybe its not worth it..
> 
> —
> Reply to this email directly or view it on GitHub <https://github.com/qutip/qutip/pull/249/files#r19719938>.
>