Added support for the `content-available` flag (fixes #30)

This adds support for the new [`content-available`](https://developer.apple.com/library/ios/documentation/iPhone/Conceptual/iPhoneOSProgrammingGuide/ManagingYourApplicationsFlow/ManagingYourApplicationsFlow.html#//apple_ref/doc/uid/TP40007072-CH4-SW24) flag to `ApnsPayloadBuilder`.