Authenticate the multiparty key if available

**Warning: This breaks backwards compatibility.**

If we have a multiparty key for the user we're authenticating, add it (and our mp key) to the secret when performing smp. (The _if_ is to remain compatible with general xmpp clients that don't support the mp protocol). Warn the user (and switch to unauthenticated) if the mp key changes (or suddenly becomes available).

This also upgrade otr.js to the latest version (v0.2.11)

Requesting lots of review before this gets merged. I definitely haven't thought through all the implications. Just appeasing @kaepora's demands in #475 and #594. This means you @vmon, @koolfy, @jcb82, @daira, @Sc00bz, etc.