guarantee emit order

 * 'part' events for fields no longer fire if `autoFields` is on.
 * 'part' events for files no longer fire if `autoFiles` is on.
 * improve readme docs
 * 'field', 'file', 'part' events are guaranteed to emit in the correct
   order - the order that the user places the parts in the request.
   Each `part` 'end' event is guaranteed to emit before the next 'part'
   event is emitted.