Support for GraphQL Interface type

[MichielDeMey]

This PR simply adds support for Interface types.

For example if you have a similar GraphQL schema

interface Device {
  ...    
}

type MotionSensor implements Device {
  ...    
}

type TemperatureSensor implements Device {
  ...
}

Device will then be picked up as a resource.

[fzaninotto]

If I understand correctly, this is also a bugfix as the current graphQL provider fails when using interfaces?

[MichielDeMey]

I would assume so. Haven't been able to test this with GraphCool or any other GraphQL services as I'm using a regular Apollo GraphQL server.

[djhi]

Thanks!