ADDED: matplotlibutilities 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.
Pushed try statement to module

try import now in matplotlib_utilities module
added dummy Normalize class
moved import of complex_phase_cmap out of try
Merge pull request #249 from nonhermitian/matplotlib_utils

ADDED: matplotlib_utilities module