Add cartopy example

[djhoese]

Problem description

Many people use cartopy to show their data in a matplotlib like figure. It isn't difficult (I assume) to put satpy data in to a cartopy figure. We may need a utility method in pyresample to convert to a cartopy transform object. With xarray DataArray's coming just around the corner this should be even easier and we can have pretty good defaults. Example should either be added to the satpy docs or to the https://github.com/pytroll/pytroll-examples repository as a jupyter notebook. @mraspaud what is your preference?

Extra: Add utility method to convert x/y-based dimensions of a resampled xarray.DataArray to longitude/latitude-base dimensions using the area metadata.