Include entity capabilities hash in outgoing presences
Also, started some work on using jsdoc for rendering API documentation.
Ideally that would go into a separate commit but that would take ages to
untangle.
And instead implement it ourselves.
This solves a bug with that plugin whereby the connection handlers are
added to early and are therefore never fired.
Also fixed a problem whereby entity items are queried for features
before the features have been fetched.
Entity items weren't being fetched from cache.
Apparently this bug only surfaced because with Ejabberd the upload
service is nested one level deeper than with Prosody.
Previously we kept all entities and their items (which are also
instances of _converse.DiscoEntity) in a flat array.
Instead, we should have a tree-like structure where items are stored
on the relevant entity (and recursively on other items).
This came out of the desire to let `converse-muc` use the API to
determine whether MUC is supported. However, we don't know the entity
JID before hand and I couldn't think of a good way to query all current
and future entities for a feature.
So `converse-muc` still does it's own thing without the API, but some
refactoring came as a result of attempting.
