added asyncReturn method

Merged #5 into here.

`bond#asyncReturn(args...)`

Should there be a method that expects the node-style callback pattern? It could automatically execute the last argument (throwing an error if it's not a function) with (null, arguments...).

It would be sugar for:

```coffeescript
bond(object, property).to (args..., callback) ->
  setTimeout ->
    callback null, someStubValue
  , 0
```