Added support for content-available flag

Currently the library only supports alert, sound and badge keys in the aps object, but [Apple's specification] (https://developer.apple.com/library/ios/documentation/NetworkingInternet/Conceptual/RemoteNotificationsPG/Chapters/ApplePushService.html#//apple_ref/doc/uid/TP40008194-CH100-SW1) also allows for a content-available key to be set. This patch fixes that issue.