Contribute Gradle plugin

Contributing the Gradle plugin [1] required
 - Updating to the latest version's `GenerationConfig` and
   package names
 - Adding to the Maven build. The Gradle plugin dependencies
   piggyback on Flyway's repository, as Gradle does not
   officially publish the dependencies for building plugins
   in another build tool.
 - Assigning copyright

[1] https://github.com/ben-manes/gradle-jsonschema2pojo-plugin
Merge pull request #123 from ben-manes/master

Contribute Gradle plugin