Expose a "getPlan()" method on QueryResult

This is very simple. We currently do not extend javax.jcr.query.QueryResult in our public API, but we have a getPlan() method on our implementation so that callers can get/see the query plan. I think we should create a org.modeshape.jcr.api.query.QueryResult interface that extends javax.jcr.query.QueryResult, add the getPlan() method, and then change JcrQueryResult to extend it (rather than javax.jcr.query.QueryResult.