Implementation and tests for URL Trigger plugin
Added documentation
Merge branch 'master' into JENKINS-18462

* master:
  Fix sendToRequester, made test to check.
  Allow an empty list of recipients.
  Add smaller git method signatures.
  Fixing TopLevelHelper tests.
  JENKINS-18396 - Add ability to inject helper classes into GroovyClassLoader used to   execute DSL code.
  Update the default values and provide convenience methods for build status values in the gerrit trigger.
  Reordered parameters according to expected most frequent usage
  Removed default value for required argument Added tests for minimal and full call to sbt method
Changed spec method name to improve readability
Added missing test for text inspection