Correct rotation handling in projections

[ryanhamley]

North was always up on Mercator projections so that a bearing of 0° meant that the map was pointing north. That's not necessarily the case for other projections. We need to create an internal rotation parameter that is disambiguated from bearing and have {get/set}Bearing convert between the rotation and the bearing based on the current map center so that we can retain the connection between bearing and real world compass directions while allowing the map to rotate appropriately based on the projection.