Add "Race" functionality

[aamironline]

This will allow user to start multiple go routine with similar with the intension to consider the result from the first go routine that has finished the task.

// GoR (Go Race) will allow will launch multiple go routines, but the result will
// be fetched from the first finished go routine!
result, err := async.GoR(fetchResource, cachedResource).Await()
if err != nil {
  log.Fatal(err)
}