Promises cleanup

Small batch of changes addressing:

* Some marginal performance improvements. Resolves https://github.com/yui/yui3/issues/571
* Adding compliance for Promises/A+ 1.1 to `isPromise`
* Deprecating `getStatus` to pave the way for YUI promises to become a polyfill for native promises

Just flexing my open source muscles. Ping @lsmith