Support for entity capabilities as per XEP-0115

I've added support for retrieving and caching entity capabilities as per XEP-0115. This lets you see which additional features the user's XMPP client supports (such as audio/video chat, multi user chats, avatars, etc.). XMPP defines these as XML namespaces (eg. "http://jabber.org/protocol/muc" for multi-user chat), the code has a list of the most common features and will convert them to "friendly" names.

Example:

```javascript
app.use(junction.capabilitiesParser());
app.use(junction.capabilities(function(handler) {
	handler.on('capabilities', function(jid, caps) {
		console.log(jid + ' supports Multi-User chat? ' + caps.features.muc + ', XHTML? ' + caps.features.xhtml);
	})
}));
```