Send JavaScript->Objective-C bridge through iframe.

I'm flailing wildly to deal with rare, inconsistent, yet persistent crashes
that seem to come from WebCore. Apparently canceling a location change in the
main UIWebView document can do bad things.
Fix compose view animation when keyboard appears.
Return an error when a quoted post has no body.

"No body" isn't the same thing as "empty body". An empty body is OK, but if the
quoted post JSON is missing a body, that's an error.