Respond to XEP-0199 pings

Has a facility to filter who we reply to, but the client doesn't use it
to filter based on the roster / presence subscriptions yet.
Initial XEP0030 support

Send <identity/>
No special types for the registry at
<http://xmpp.org/registrar/disco-categories.html> yet.  May wish to add
that.
Optional LangTag for <identity/>
guard was backwards
Ability to add features to Disco

Nodeless info disco is now done.

This also gets Ping to add its feature.
We do this now
Ability to filter disco responses by Jid of from
Ability to ping out and measure latency

Ping returns either timeout, error, or time
Main currently pings singpolyma.net every 30 seconds and prints the
result