Fix RMMapBoxSource initWithMapID crash when offline

This change fixes `RMMapboxSource` `initWithMapID:` functions crashing when the user is offline (https://github.com/mapbox/mapbox-ios-sdk/issues/259). These init methods will now return nil instead, if the user is offline.

Also included in the pull request are NSError parameters which are changed from nil to NULL (https://github.com/mapbox/mapbox-ios-sdk/issues/287), and a replacement for the deprecated `NSPropertyListSerialization propertyListFromData` call.