We should remove XMPP-terminology

[zoni]

Err started out as a bot for XMPP only, which shines through in various parts of the back-ends.

However, things such a Jabber IDs (JID), nodes and resources don't make sense in the context of anything other than XMPP. It would be nice if we could remove these concepts from non-XMPP back-ends somehow.

I'm thinking we should expose some generic methods on Identifier classes. Something like a username property that returns the username in the preferred format the back-end supports.

For XMPP this could return the JID, HipChat and Slack could return the username (that one that is used with @-mentions), IRC the username, etc.

We would also need to remove the rigid concept of node, resource and domain. I'm thinking Identifier objects should let node, resource and domain be optional, instead of always being there on all back-ends like they are now, letting back-ends be smart enough to only use what they need (and makes sense for their network).

The important thing to do however is to make sure Identifiers are simple to work with for plugin authors, so that they can send messages to/from one, in a way that works consistently for all back-ends so that they don't need to know about any specific network quirks.

[gbin]

This is done