SLING-4206 Refactor Sightly and Sightly JS

 - Cleanup API
 - Structure API
 - Move implementations into impl folders
 
Applying patch by Radu Cotescu (thank you very much)

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1642281 13f79535-47bb-0310-9956-ffa450edef68