Updated LICENSE and LICENSE headers

This updates the root POM to use the file `LICENSE_HEADER.txt` for the license headers. It also adds that file, taken from the Apache licensing tutorial. Also, `LICENSE.txt` is updated accordingly as well.

All other changes stem from running `mvn license:format` and are purely automatic.

This addresses [REEF-26](https://issues.apache.org/jira/browse/REEF-26)