Add a getEntryNames() method to POIFS/NPOIFS directory entries, to make listing easier

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1590148 13f79535-47bb-0310-9956-ffa450edef68
Refactor some of the NPOIFS write tests, and begin on the last missing write test

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1590149 13f79535-47bb-0310-9956-ffa450edef68