Add definitions for new settable zone function

Moment.js version 2.1 added a new zone function which takes either a number or string and returns a Moment, see here: http://momentjs.com/docs/#/manipulating/timezone-offset/ . So the definitions for that function should be added to this file.
Merge branch 'patch-1' of https://github.com/gscshoyru/DefinitelyTyped into momentjs-zone-function
definition fix
Merge pull request #1245 from borisyankov/momentjs-zone-function

Momentjs zone function