Implement get${Resource}AfterCreation methods

[mdb]

When a resource is created, the VinylDNS API returns a 202. It's the client's responsibility to poll the resource's URL until the client receives a a response with an HTTP status code of 200 (rather than a 404, which may be returned during the window of time in which the resource is being created).

It may be useful to implement helper methods such that users aren't required to write their own polling. For example, a getZoneAfterCreation(zoneId, optionalTimeout) method could poll getZone(zoneId) and resolve the promise it returns once getZone(zoneId) no longer returns a 404.