Configurable jbake version

The version of jbake is now configurable via extension:

```groovy
jbake {
  version = '2.3.0'
}
```

The engines versions are configurable too :)

The dependencies for jbake and its render engines are added by the plugin dynamically.
See JBakePluginSpec and visit the README.adoc for further information.

Should fix issue #6