Add support for appengine/delay

fixes #10
I tried to figure out something clever so you could do:

```
var df = delay.Func("key", appstats.FromContext(func(c appengine.Context, args ...interface{}) {
  // stuff
}))

// in appstats.go:

func FromContext(f interface{}) interface{}) {
  // make context, time it, and invoke f()
}
```

But I didn't want to deal with the reflection for invoking f as a function. I thought this WithContext implementation is simple and easy. Also it can work with other things that aren't the delay package.