Add onReconnect() callback to EngineListener to allow jenkins to
hook into a slave reconnect attempt.
Forms the basis of a fix for JENKINS-24272
"jnlp slaves fail to reconnect when master is restarted"
Add since to onDisconnect()
Merge pull request #25 from oldelvet/jenkins-24272

JENKINS-24272 Add onReconnect() callback to EngineListener