JSONP async fix and tests

This is the proper fix for #371

This adds `Y.Get.js().execute()` support.

I also added proper tests that work with `echoecho` under `yogi test` too.

_Note: this should be tested with `yogi@0.0.63` to make sure you get the latest `echoecho`_

Ping: @lsmith @rgrove @ericf