polar stereo grib

Allows polar stereographic grib files to be loaded and plotted with Iris.

Requires [Cartopy PR 275] (https://github.com/SciTools/cartopy/pull/275) for updated polar stereo projection.
Requires [test data pr 3] (https://github.com/SciTools/iris-test-data/pull/3) for test data.

Also pending a PR from @ajdawson for better missing data handling.