Added integration test for generated protoc plugin

I have added an integration test for protocPlugin support. The setup-protoc-plugin project builds and installs a minimal plugin written in Java. This is referenced in TEST-16. 

The setup-protoc-plugin depends on protobuf Descriptor and Plugin protos and protoc-generated sources for these have been added to src/main/java for simplicity. I don't know if it's possible, or even desirable, to have them generated by maven-protoc-plugin itself.