function/after

As described in issue #138 this adds a utility called `after(fn, delay, [context])`

It enforces a callback to be delayed for a minimum amount of time, yet after this delay it will reach through immediately. 

It's a little hard to describe. @conradz offered to help me out with the test (and maybe the doc?). I tried to make the as similar as I could to other asynchronous tests such a `function/debounce`. It would like to take you up on that @conradz.