[appboy] Pull Request #25: If a message download is in-progress, wait until complete before logging a custom event

[SegmentDestinationsBot]

Migrated from #25 by @hmedney

Basically this keeps track of the callback in a window.appboy scoped var and provides a new function called _messagingReady(callback). Calls to logCustomEvent() can be wrapped as follows:

this._messagingReady(function() {
  window.appboy.logCustomEvent(eventName, properties);
})

When the messages have been downloaded, the callback is invoked. In testing simple pages, it seems to solve the race condition and make the integration more predictable.

For example, if a page instruments as follows:

analytics.load("{writeKey}");
analytics.identify("{userId}")
analytics.page();

The above change will ensure all the Braze in-app messages have been downloaded before the page custom event is called on the SDK.