Matplotlib data is installed into the matplotlib module.
 This is similar to package_data.  This should get rid of
 having to check for many possibilities in _get_data_path().
 The MATPLOTLIBDATA env key is still checked first to allow
 for flexibility.

 1) Separated the color table data from cm.py out into
 a new file, _cm.py, to make it easier to find the actual
 code in cm.py and to add new colormaps. Everything
 from _cm.py is imported by cm.py, so the split should be
 transparent.
 2) Enabled automatic generation of a colormap from
 a list of colors in contour; see modified
 examples/contour_demo.py.
 3) Support for imshow of a masked array, with the
 ability to specify colors (or no color at all) for
 masked regions, and for regions that are above or
 below the normally mapped region.  See
 examples/image_masked.py.
 4) In support of the above, added two new classes,
 ListedColormap, and no_norm, to colors.py, and modified
 the Colormap class to include common functionality. Added
 a clip kwarg to the normalize class.