Patch from Sofia Larsson and Martin Andersson from bug #56020 - XSSF support for creating Pivot tables

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1614684 13f79535-47bb-0310-9956-ffa450edef68
Add row label getter for Pivot Tables, and use this in unit tests

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1614691 13f79535-47bb-0310-9956-ffa450edef68