Cube variables should have basic image display information

[forman]

In order to display cube subsets as images, each variable shall be associated with a default colour map and min/max display range in the variable's units. The colour maps may be defined by name and if so, shall be compatible with matplotlib colour map names.

See also:

• http://matplotlib.org/users/colormaps.html
• http://matplotlib.org/api/colorbar_api.html
• issue #18