SLING-3423 / SLING-3657 - adding MergedResourcePicker for implementing additional merged resource selection algorithms. Implmeneting OverridingResourcePicker to provide /mnt/override which merges resources based on the resource type hierarchy

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1622390 13f79535-47bb-0310-9956-ffa450edef68