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
Don't remove window.__synthTest

In Firefox JavaScript inside an href (for example:
    `href="javascript:__synthTest()"` fires well after Syn has loaded.
    Instead of trying to predict when it is "safe" to remove this
    function, we're just going to leave it alone as it is double
    underscored and shouldn't conflict with anything. In the future we
    could create a `noConflict` function that removes this. Fixes
    lingering issue in #30