Should there be an event when the adapter is "ready"?

[kellyselden]

Stemming from #151 and #86. They are both having issues with race conditions waiting for the upstream script to finish, and what to do with trackEvent calls while it's loading. One option is for the metrics adapter to extend Ember.Evented, and adapters could trigger an event that anyone can listen to. It could be optional, an adapter could choose not to send it.

An adapter could still do its best to queue trackEvent calls while the scripts are loading. The benefit of this change is it would be way easier to test adapters, and a host app may find use out of a hook that fires when the adapter is ready.