REEF-38: 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

**Known issue: ** RAT doesn't pass quite yet because somehow the exclusion of ".git" is ignored by it. We'll have to resolve that later.
Hi @markusweimer  I don't think you need a header on the README? You can just tell RAT to exclude these files.