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

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.

Start in the appboy integration and trace how message downloads and logCustomEvent() are currently sequenced. Review the migrated Pull Request #25 for the proposed callback behavior, then verify the analytics.load(), identify(), and page() sequence. Done means custom events wait until Braze in-app messages finish downloading.