REEF-36: Add Apache RAT
  * Added RAT
  * Added license headers to poms
  * Removed eclipse plugin for which no license was available
  * Added a missing licensing header in the .proto files
  * Added a missing licensing header in the .avsc files

This closes#14

This is to resolve issue REEF-38 https://issues.apache.org/jira/i#browse/REEF-38?jql=project%20%3D%20REEF%20AND%20resolution%20%3D%20Unresolved%20ORDER%20BY%20priority%20DESC

Author: Markus Weimer <weimer@apache.org>