Promise-based validation example and best practices?

Hey,

Thanks for the great library. I've noticed that you can return a promise from a custom validator leading me to believe I could write validators that query a remote backend. The most common use-case for this would be checking for other instances of a model with the same value (unique email for example).

It would be great to see a more verbose example this. I'm not sure what is the best way of getting access to the data `store` as an injected dependency. From my understanding (based largely on [this SO answer](http://stackoverflow.com/a/34971940/396300)) you would need to declare the validator as an Object and register it with the container to be able to inject the store dynamically.

I may be overthinking this but would like to hear about some best practices when needing to interact with other models during validation.