Implement XEP-0316: MUC Eventing Protocol

As discussed during the XMPP Summit, MEP (PEP over MUC) would be a nice addition to allow adding a couple of nice features on top of MUC including pinning messages (a XEP that I'd like to write when MEP is implemented).

It would then also be nice to have room vCard and room Avatar this way (instead of the vcard-temp "hack").

Thanks again for the awesome work :) !