Merge pull request #108 from tkrajacic/master

Fix #109, crash due to bad null pointer de-referencing logic
refs #111: make attribution more prominent
refs #105: no need to defer calls already on the main thread
refs #85: flesh out MBXPointAnnotation docs & properties
refs #85: improvements to MBXOfflineMapDownloader docs
refs #85: constant/type docs cleanups
refs #85: category docs improvements
refs #85: improve docs for MBXRasterTileOverlay
move API to private
refs #85: clean up MBXOfflineMapDatabase docs
refs #85: backtick formatting tweaks
refs #85: few more formatting tweaks
refs #85: fix typo
refs #85: fix back out of coordinate setter/getter
fix header comments
refs #85: improvements to MKMapView category
fix some spacing
safely prefix category method name
refs #82: consolidated user agent
rename NSError methods with prefixes like other categories