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.*}

Start by inspecting MessageType and the Type class hierarchy to determine which schema classes need serialization support, then look for existing serialization tests in the repository. Done means the relevant schema objects can be serialized for ParquetRDD use without breaking existing behavior, with tests covering the result.

Written by the indexing model from the issue text.