Bugfix: EIOPromise::Create was allocating two EIOPromise objects

This is because it would call the javascript initializer which executed
Promise::New, and then it would rewrap the handle. Instead I make an
explicit inheritance from EIOPromise to Promise.

This seems to fix a memory leak which was reported by Ray Morgan:
http://groups.google.com/group/nodejs/browse_thread/thread/e38949b1989da1d7