content-available support

For iOS7 remote-notification background fetch (application:didReceiveRemoteNotification:fetchCompletionHandler)

{ content-available: 1 } must be in aps hash.

https://developer.apple.com/library/ios/documentation/NetworkingInternet/Conceptual/RemoteNotificationsPG/Chapters/ApplePushService.html

see Table 3-1 Keys and values of the aps dictionary



Example Usage:

```
          APN.notify_async(token, { alert: alert,
                                    data: data,
                                    content_available: true,
                                    sound: 'default'
                                   })
```


NB. APNS expects the dashed name ['aps']['content-available'], however snake_case is used to preserve the simple use of ruby keys.