Running unit tests for UnpackedAvroSource result

This pull requests reproduces the issue I'm experiencing with testing `AvroUnpackedSource` via `JobTest`.

In sbt, you can run the test via `test-only com.twitter.scalding.avro.AvroUnpackedSourceTest`, resulting in  a `java.lang.IndexOutOfBoundsException: Index: 12, Size: 1`.

I included the generated Avro classes to avoid having to integrate Avro into the sbt build.