Bumped liverebel-plugin-util dependency, to include a fix for a NPE when
liverebel.xml is not provided and override information is not specified.
The fix is to give a proper error message, instead of NPE.

Also, refactored logging to use SLF4J API (and delegate to correct
implementation in the background, either build job console or Jenkins
internal java.util.logging).

Finally, added a checkbox to LiveRebel Jenkins plugin configuration
page, to make it easy to turn debug level logging on and off.