Updated Q.async to work with current ES6 generators without send()

Generators in the current ES6 spec [no longer have a `send()`](https://bugs.ecmascript.org/show_bug.cgi?id=1555) method, it's replaced with with `next(value)`: the `next()` method called with a value.

Updated the Q.async implementation to work with current ES6 spec (version in node 0.11.3 with --harmony-generators).