Provide projection-aware tick formatters.

This PR adds projection-aware tick formatters for latitude and longitude axes. These fit with the current tick labelling capabilities, designed only for rectangular projections. They work by transforming the tick values to the `PlateCarree` projections before doing the formatting, resulting in meaningful tick values. The intention here is to provide something that "just works" for the most common cases (see #204), but future efforts may be better off going in the direction of improving the grid line labelling abilities of cartopy.
Good idea. I'll implement this and ping you back when done.