Added the option of showing Maidenhead grid squares on the World Map, and the option of shading in worked grid squares. Addresses issue #59. Note that this introduces a new class called Maidenhead, which is capable of converting between latitude-longitude coordinates and grid squares. However, this functionality isn't currently used.
Migrating grey line functionality over to a Cartopy-based implementation to address issue #62.
Also renamed GreyLine to WorldMap, and bumped the version to v1.1.0-dev.