CB-2530 and CB-2239 Multipart plugin result

This patch does a bunch:
* [ios&android] Adds support for multiple argument callbacks for
  win/fail (by way of updating callbackFromNative)
* Adds MultiPart message type, which supports an array or return
  arguments from a plugin result (and will massage each argument, such
  as ArrayBuffers)
* Update Echo plugin to test MultiPart messages