Fix for #22: Only write to cache for an HTTP 200, sweep empty png files.
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.
Fixed not setting hasInitialCenterCoordinate when manually setting the map camera
update podspec email
fixes #17: point CocoaPods docs at mapbox.com
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
prefer properties to straight getters
Merge branch 'master' into patch-1
minor formatting
refs #23: minor cache fix cleanups
 * define constant error domain
 * cleanup some comments
 * avoid early return
 * valid HTTP responses should already check for status 200
Merge branch 'master' into 23-cache-fixes
more cache response fixes based on returned error & HTTP status
fix response cast, handle non-HTTP responses, and define error code