Overhaul MELS to deal with causality, kicks, etc.

The MELS can now deal with arbitrary sequences of transitions per user, where a transition is a change in membership. A transition can be joined, left, invite_reject, invite_withdrawal, invited, banned, unbanned or kicked.

Repeated segments (modulo 1 and 2), such as joined,left,joined,left,joined will be handled and will be rendered as " ... and 10 others joined and left 2 times and then joined". The repeated segments are assumed to be at the beginning of the sequence. This could be improved to handle arbitrary repeated sequences.

src/components/structures/MessagePanel.js

@@ -281,7 +281,6 @@ module.exports = React.createClass({
 
         var isMembershipChange = (e) =>
             e.getType() === 'm.room.member'
-            && ['join', 'leave'].indexOf(e.getContent().membership) !== -1
             && (!e.getPrevContent() || e.getContent().membership  !== e.getPrevContent().membership);
 
         for (i = 0; i < this.props.events.length; i++) {