NF - option to make colorbar extensions rectangles

[ajdawson]

I made a small change to allow the minimum and maximum colorbar extensions to be rectangular instead of triangular. This makes most sense when used in combination with the recently added extendfrac keyword with extendfrac='auto'.

A use case for this (one I recently encountered) is the need to make plots from matplotlib visually consistent with plots produced in other systems. For example, the NCAR Command Language (NCL) cannot produce triangular colorbar extensions at all (see http://www.ncl.ucar.edu/Applications/labelbar.shtml).

I appreciate that the same result could probably be achieved through fiddling with the colorbar values and labels, but this feels like the most intuitive solution for users.

[WeatherGod]

I like the idea, and your use-case is certainly valid as I know many people would want to have as identical as possible images when porting a script from one language to another. Just a few things for polishing: put asterisks around "True" and "False" in the docstrings (and make sure to use "True" and "False" instead of "true"/"false"). Add a test. Add an entry to doc/users/whats_new.rst. Note that you will likely have to create a new section for v1.3.

[ajdawson]

I've added tests, actually I refactored the test_colorbar.py module somewhat to accomodate the new tests more efficiently.
Also added entries to doc/user/whats_new.rst and doc/api/api_changes.rst.
Finally, rebased onto master

[ajdawson]

I think this one is ready to go now. It is freshly rebased onto master ready for merging.

[WeatherGod]

Thanks for the ping. Things look good to me here. Merging!