Y.Deferred, yo.  Let's make this happen!

Adds Y.Deferred and Y.Promise in 'deferred' module and features and Y.when (like Y.Parallel) in 'deferred-extras'.

Aimed to serve as the standard API for a transaction object/layer for big IO refactor.  API feedback still welcome, though there was much discussion https://github.com/lsmith/yui3/pull/36.