Adding licence headers

https://issues.apache.org/jira/browse/USERGRID-4

I'm adding license headers of files (java, xml, properties, etc). Also adding apache-rat maven plugin.
Overall review, please. However, we have to avoid license conflict in Inflector class ( remove or replace something )
https://github.com/usergrid/usergrid/blob/master/stack/core/src/main/java/org/apache/usergrid/utils/Inflector.java