Call parent window from iframe #30

Fixes the test that was failing in Firefox for #30. The old test was waiting for the iframe to load and then setting a function on the iframe's window. But Firefox would call `load` after steal had completed, so it was unreliable. The fix is to set the function on the parent window and have the iframe call the `parent.synReady()` function. This guarantees that the function exists before the iframe script has finished running. Should close out #30