Provide projection-aware tick formatters. Closes #204.
Make GeoFormatter class public and revise test procedure.
Simplified formatter class structure.
Made shared parent class completely private.

Also added notes to docstrings about reuse of the formatter
classes.
Modified the precision of transformed tick locations.

Tick locations are rounded after transformation to prevent spurious
values from making the plot look bad. The precision to which they
are rounded is increased, and can be overridden by the user.
Clarified limitations in class docstrings.
Match exception messages in error tests.
Added examples in docstrings.
Added an example and a what's new entry.
Fixed PEP8 error.
Removed duplicate entry in "Whats's New".
Class variable for target projection since it never changes.
Merge pull request #401 from ajdawson/geo-tick-formatters

Provide projection-aware tick formatters.