Added a files needed to upload Pig to Maven Central

At some point in the future this will have to change because `android.sourceSets.main.allJava` is not accessible anymore in the lastest version of the android plugin. So we will have to use the task above that uses `variant.javaCompile.source`. However, I'm not sure how to link this to the artifacts because there will be different tasks for every variant. We can leave it like this for now.