PARQUET-93: Add support for protobuf to parquet-cascading

The parquet-cascading module currently includes code that lets Cascading write and read Thrift-based data to Parquet files. Protobufs is another popular alternative to Thrift which can be explicitly supported by leveraging the existing code in parquet-protobuf.
Without this change, the compiler does not notify folks that you can't use a ProtoParquetWriter to write any old object (I know, because I did it by accident once...).  In other words, if you don't override the method, it has a method signature of "void write(Object obj)" when really that obj parameter needs to be something that extends MessageOrBuilder.