SLING-3148 Not only look for the sling:features property in the resource itself
   but also in the jcr:content child node.
   
This allows JCR based applications to manage the sling:features property in the
application level document's jcr:content root.

Also moved the getProperty method as a static method into the
ResourceResolverContext class to make it available as a general
utility method.

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1562011 13f79535-47bb-0310-9956-ffa450edef68