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
I'm curious how this works. What calls `getInstallers`?