Bug 1043971 - Implement specific event for Loop calls "on hold"

See bug https://bugzilla.mozilla.org/show_bug.cgi?id=1043971 please.
I think we dont need this. We are lazy loading https://github.com/mozilla-b2g/firefoxos-loop-client/pull/92/files#diff-d583766359512ce43e01fe67d7d3907fR69 some lines after. Probably it would be enough to use a condition https://github.com/mozilla-b2g/firefoxos-loop-client/pull/92/files#diff-2fbba884ef4a966212c06b83fa8ebd85L30 in order to check if it's an incoming/outgoing call. Wdyt?