Protect attempts to reference the parent frame.
Protect attempts to reference the parent frame.

This causes cross-domain access exceptions when embedding
CodeWorld into another web page.  We catch that exception,
and offer a fallback.

Fixes #552

pranjaltale16/codeworld,tgdavies/codeworld,google/codeworld,alphalambda/codeworld,pranjaltale16/codeworld,tgdavies/codeworld,alphalambda/codeworld,pranjaltale16/codeworld,google/codeworld,tgdavies/codeworld,alphalambda/codeworld,google/codeworld