As Jdk 1.5 doesn't provide NavigableMap/Set classes of JDK 1.6, we temporarily use the open jdk classes instead, until POI has switched to JDK 1.6

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1561500 13f79535-47bb-0310-9956-ffa450edef68