SLING-2944 Implement Service User Mapper

* Add ServiceUserMapper service and implementation bundle
* Add service login methods to ResourceResolverFactory
  and SlingRepository
* Add implementations of new methods

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1510413 13f79535-47bb-0310-9956-ffa450edef68