Fix #700. Add basic {callback} replacement support back into jsonp-url's `_format` method

I needed to refactor all the tests to remove the Y.Get dependency. The jsonp test suites shouldn't be testing Y.Get, so I stubbed it locally for each test (restoring the original in `tearDown`.

I also made the jsonp-url test page load both jsonp-tests and jsonp-url-tests to verify the tested core behavior continued to work when jsonp-url was loaded.

Though it's not likely visible in the diff view of the changed test files, I added a test for this issue as well.