Add computer, queue listeners

[llaaiiqq]

Fixes #5

Changes proposed in this pull request:

• Added computerListener to track node activity
• Added queueListener to track jobs in the queue
• [X] Includes tests covering the new functionality?
• [X] Ready for review
• [X] Follows CONTRIBUTING rules

@nickgrealy

[nickgrealy]

Nice commit, the changes are now live - version 1.014...

• https://github.com/jenkinsci/groovy-events-listener-plugin/releases/tag/1.014
• http://updates.jenkins-ci.org/update-center.json

Thanks @llaaiiqq!

[llaaiiqq]

Thanks for the feedback, and thanks for picking up the change!