Support the Mapbox image quality API.

MBXMapView now offers a `sourceQuality` property that can be set to
one of the MBXMapKitSourceQuality values. These values map to the
file extensions defined by the Mapbox image quality API.
update iOS codesign stuff
Merge branch 'master' into source-quality
caps & copyright
Merge branch 'master' into source-quality
slight tweaks to #27 image quality API