when-promisify

Make any node-style callback function return a promise instead.
when-promisify supports two ways to promisify a callback function.

1. simple way: just pass a function

   var promisify = require('when-promisify')
   var promise = promisify( cbFunction )

2. pass a function with its context

   var promisify = require('when-promisify')
   var promise = promisify( Object, 'methodName' )