Use more compact encoding than base64

[asn-d6]

We can do better than base64. We can have a more compact encoding, specifically tailored for IRC or XMPP.

For example, as far as I know, IRC servers are happy to transfer binary data as well, except from a few values (I think there are 3 disallowed values). Which in theory could even allow us to do base253. I think Joe has written a base253 encoder somewhere but I can't find it now...

We should consider what kind of encoding we want and try it out.

(I wonder why other protocols like OTR are not doing this as well? Maybe it's a good idea to be conservative and stay with base64 to support more chat frameworks?)