Putting the idea plugin back in the gradle build

Hey Justin / guys,

I finally have managed to get a little time to put into this and am starting small, going back through the stuff on the wiki.  i noticed this was missing so put it back in.

...radlew cleanIdea' and './gradlew idea' commands described in https://github.com/jenkinsci/job-dsl-plugin/wiki/Building-the-Jenkins-Job-DSL.

Cheers, Andrew