Otherwise we will reuse old/stale objects for which we receive no add events
(resulting in an empty roster).

Looks like the subsription flow was broken since the roster groups refactor.
Requesting and pending contacts weren't being properly added to (and removed
from) roster groups.

Requesting contacts are now properly added and remove. Next up, handling
outgoing requets (i.e. pending contacts).

Chats were being trimmed even though there was enough room.

When loading a page with an existing session, the roster view's elements were
being added too early, resulting in event handlers not being properly
registered.

and thereby fix bug with directInvites.

When re-attaching to an existing session, chat rooms are fetched from
sessionStorage and we join them again.

However, unless we send a presence of type unavailable before reloading the
page, from the XMPP server's perspective we never left the chat room.

It therefore doesn't send us again the room occupants or room messages.

To send a presence of type unavailable is a hack and not desireable. Rather, we
want to stay in the room and just re-attach to it upon page reload.
In order to do this, we need some new functionality.

* Refactor the chat room sidebar into a new Backbone Model/View combo. (done).
* Store/fetch room occupants in/from sessionStorage (done).
* Store/fetch room messages in/from sessionStorage (not yet done).
* Instead of re-joining a chat room which we never left, just register the event handlers again. (not yet done).

When calling require from within HTML, then don't use data-main, but instead
load main.js via a require call.

See here: http://requirejs.org/docs/api.html#data-main

Makes it easier to integrate converse.js into other bower/require projects.

onChatRoomPresence and showStatusMessages have been refactored to give more
detailed information and to also show the reasons given to actions taken by
moderators.

Also document the available chat room commands.