Add check-for-update and apply-update to iframe-install API (bug 1097430)

https://bugzilla.mozilla.org/show_bug.cgi?id=1097430

Needed to be able to tell whether a Marketplace update is available (and then download it) from Marketplace itself.

Added (temporary, need a better place to live, like marketplace-frontend) docs here: https://github.com/mozilla/commonplace/wiki/iframe-install-messaging-API
Yeah, that's probably a good idea, but I just wasn't sure what do say in the message exactly... It's already pretty big. Suggestions ?