Support whether the returned value was cached or not

There is no way to know if a returned value was the result of the call or a cached result? This is useful to know in certain circumstances.

Something like:

``` go
// SomeMethod ... and also returns whether the value was cached result or not
func (g *Group) SomeMethod(key string, fn func() (interface{}, error)) (interface{}, bool, error) {
```

In the case of cached lookup the returned bool is true.