Add Q.isPromise method

Hello @nicks, 

Please review the following commits I made in branch 'kyle-ispromise'.

302659998c1dd9f80695211ec075660050dfbfda (2013-07-29 16:12:44 -0700)
Add Q.isPromise method


R=@nicks
VISIBLE_CHANGES=Add Q.isPromise method to kew