Added Bing Service.
Removed superfluous registration method in favor for existing registration method by allowing nil to be passed into registerService:consumerKey:subscription: method.
Added more consistent code formatting.
Renames constants and minor typo on documentations
Updates missing constants
Updates deprecated APIs from SDWebImage 3.7
Enables AFNetworkActivityIndicatorManager
Adds Bing conversion from service name, fixing bug causing not being able to have multiple services allowed.
Adds more attributes to metadata model for Bing and Google Images
Adds contentType property to metadata model
Sets default services for sample project
Includes content_type for resulting payload. Minor refactoring.