Allow immediate resolution upon Deferred instantiation

Allows simplifying patterns like so:

```coffeescript
cachedValue = null
foo = ->
  if cachedValue?
    return (new Deferred cachedValue).promise
  d = new Deferred
  $.get('/some/slow/process').done( (data) ->
    cachedValue = data
    d.resolve data
  )
  return d.promise
```