Implement opensearch suggestions in the API (bug 898020)

https://bugzilla.mozilla.org/show_bug.cgi?id=898020

This is an experimental feature, we probably don't want to expose this in the docs till we are certain that this is the right format (*). This is designed to be used by gaia but also potentially for search suggestions in the marketplace itself if we want to re-enable this (see https://bugzilla.mozilla.org/show_bug.cgi?id=847625)

(*) In particular, we are extending the spec here with a 4th array containing icons. The spec has not changed in years, so it should not be a problem as long as every browsers ignore extra arrays they don't know about (they should)