Added some nullability qualifiers in method implementation selectors for consistency with declarations.
The user dot view now has a non-zero size. Previously only its layer was sized properly. Also resolved some pedantic warnings about missing designated initializer overrides.
Xcode 6.3 or above is now required for building this project.
* Cleans up & documents more APIs, especially in location.
* Moves `coordinate` internal and makes use of `location` to retrieve it.
* Moves `title` & `subtitle` internal since part of `MGLAnnotation` protocol.
* Enables `isUpdating` by privately associating weakly with a map view.
