add a new XMPP service hook to eventually replace the jabber hook

Instead of sending all hook data for *all* of GitHub to poor jabber.org's server
I suggest that since the person receiving the hook already has an XMPP server
running their bot, that the sending JID should be defined by them.

This is a copy of what was in jabber.rb but with JID and Password added and
also support for receiving Issue events.

I tried to test this locally but following the directions to run a local copy of the github-services failed - the docs don't match what is in the repo.  I'm willing to fully support this including setting-up whatever environment is required to test this.