Offer exporting jar of jdom.contrib

I wanted to use `org.jdom2.contrib.dom.DOM.wrap` as mentioned in an answer to the question [Is there any org.w3c.dom wrapper for JDom?](http://stackoverflow.com/a/10210571/873282) on stackoverflow. For that, I required a jar file. I added a (gradle)[http://www.gradle.org/] build file to enable creating a jar (`gradle jar`) and uploading it to the local maven repository (`gradle install`). I did not add the [Gradle Wrapper](http://www.gradle.org/docs/current/userguide/gradle_wrapper.html), so gradle binaries are required.