Update gradle plugin with upstream changes

[rravi-sift]

Summary of change

1. Checked out https://github.com/joelittlejohn/jsonschema2pojo/tree/master/jsonschema2pojo-gradle-plugin
2. Copied the contents of jsonschema2pojo-gradle-plugin from the original repo to this repo
3. Reverted the new parameter changes in JsonSchemaExtension that are not in this core library.
4. Add mvn config to build fat jar in jsonschema2pojo-gradle-plugin
5. Build fat jar

mvn package -Dmaven.test.skip

6. Deploy the gradle plugin to the internal registry

mvn deploy:deploy-file -DgroupId=org.jsonschema2pojo -DartifactId=jsonschema2pojo-gradle-plugin \
  -Dversion=<version> \
  -Dpackaging=jar \
  -Dfile=<jar_location>-jar-with-dependencies.jar \
  -Durl=<repo>

[nicolesu]

lgtm thanks @rravi-sift