remove sniff for Opera

[miketaylr]

https://github.com/cowboy/jquery-postmessage/blob/master/jquery.ba-postmessage.js#L67

You probably couldn't get it to work because postMessage() used to be on document. Opera's implementation predated the specification of window.postMessage().

Would be nice to fix this, because it's unusable in Opera today--history is clobbered.

[miketaylr]

Oh, just checked and 9.64 has postMessage on window. I can peek more next week or so.