[Feature Request] jbundler init

[coderhs]

I was just starting with JRuby and jbundler, was starting a project. Assuming that jbundler might be having similar commands as bundler i tried jbundler init and found it to be missing. I feel it would be a good command to have in jbundler. We can do it such a way that, when someone does jbundle init it would create the Gemfile through bundle init and then create an emtpy Jarfile with example on how to add the dependency.

If others are also open to this idea, then I would add this feature and send a PR.