Make traceur.runtime.StopIteration into a constructor function.

Make traceur.runtime.StopIteration into a constructor function. This is a stub until I figure out how to proceed with this. The global StopIteration remains an object, but it can't be the browser-supplied one. Firefox's StopIteration singleton object: - Cannot implement the return semantics of harmony:generators. - It's impossible to create objects x such that x instanceof StopIteration Since a normal JS object cannot be the rhs of 'instanceof', a traceur-supplied StopIteration cannot be used that way. This breaks Task.js (unless we patch it).