Support for APNS alert dictionary schema

Currently the `alert` only supports a single string field, however the APNS `alert` field can be expanded to be either a string OR a dictionary as in:

```
aps: {
alert: {
title: "some title",
body: "some body",
...
}
}
```

As documented here:
https://developer.apple.com/library/ios/documentation/NetworkingInternet/Conceptual/RemoteNotificationsPG/Chapters/ApplePushService.html#//apple_ref/doc/uid/TP40008194-CH100-SW1

Our immediate need is for `title`, `body` and `launch-image` support, but it might make sense to handle all of the localization keys as well while you're at it.

No file or test is named in the issue. Start by tracing how the APNS alert payload is assembled and where the current string-only alert is handled. Done means accepting either a string or a dictionary and supporting title, body, and launch-image; decide whether localization keys are included.

Written by the indexing model from the issue text.