jbake 2.3.1

Binary java package with a formula similar to that of Grails or Groovy.
We have a helper to do this in one step:

https://github.com/Homebrew/homebrew/blob/1abb2b04e24bde05a169f8ab0e65dcc1a0247179/Library/Homebrew/extend/pathname.rb#L388-394

```rb
bin.write_jar_script "#{libexec}/jbake-core.jar", "jbake"
```