Make MessageType serializable

Background: I'm writing `ParquetRDD` implementation to run on Apache Spark. One of the features I want to provide is the ability to create instance of `ParqeutRDD` with a projection (specify columns to be loaded)

Problem: Schema information is not serializable. Neither `MessageType` nor any other class that extends `Type` extends `Serializable`.

Question: Is there any specific reason why not to have it the `extends Serializable`?
If not, then I will be more then happy to provide PR.

**Reporter**: [Pawel Szulc](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=rabbitonweb)

_{**Note**: *This issue was originally created as [PARQUET-850](https://issues.apache.org/jira/browse/PARQUET-850). Please see the [migration documentation](https://issues.apache.org/jira/browse/PARQUET-2502) for further details.*}