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.
I'd consider doing this in ```__init__``` and assigning to `self._target_projection` or similar to save instantiating a new object on every call.