Add CRS accessor for cartopy

[aulemahal]

This fixes #395.

• Add a crs accessor to both the Dataset and DataArray accessors.
  • For Datasets, we can only return something if only one grid mapping is defined.
  • For DataArrays, the grid mapping must be a coordinate
  • For both, the absence of a grid mapping and the presence of a "longitude" entry in the dataset is assumed to mean that the grid mapping is "latitude_longitude". CF allows an absent grid mapping for this CRS.
• Include this in the plotting function (?)
• Add tests

[dcherian]

Also, +1 to automatic use in the plotting function, that was my initial intent!