Add license headers and enable apache-rat-plugin.

The RAT plugin checks for any files that do not have a license header
and fails the maven build. This helps ensure that all files are
correctly marked as they are added to the repository.
Adding NOTICE, DISCLAIMER, and KEYS.

NOTICE: All parquet code was contributed to The Apache Software
Foundation, so the NOTICE is the simple version.

DISCLAIMER: Recommended disclaimer with "Apache Parquet" as the project
and "The Apache Incubator" as the sponsor.

KEYS: Generic KEYS header, no keys yet.