PARQUET-72: Prepare for Apache release

Add license headers and other documentation required by the ASF.

This doesn't update the maven release configuration.

Author: Ryan Blue <rblue@cloudera.com>

Closes #6 from rdblue/PARQUET-72-prepare-apache-release and squashes the following commits:

e48a607 [Ryan Blue] Adding NOTICE, DISCLAIMER, and KEYS.
3d2ca06 [Ryan Blue] Add license headers and enable apache-rat-plugin.