jruby-gradle-jar-plugin is walking upright

In this changeset:
+ Jar task type extended with `jruby` closure
+ Ability to add custome gem directories
+ Ability to make a JAR executable using `warbler-bootstrap`
+ Ability to make A JAR executable supplying own main class
We let Gradle do the magic here. Effectively that gets passed to `from(dir)` which allows you then to do something like `from({project.jruby.gemInstallDir})`, delaying the evaluation until the very last moment!