Improvements to temporary file handling

A set of commits that allow to override the way temporary files are created (see https://issues.apache.org/bugzilla/show_bug.cgi?id=56735).

There are two small API changes related to what I see as possible race condition vulnerability when creating temporary files:
 - org.apache.poi.util.TempFile.createTempFile() now throws an IOException
 - I deleted the org.apache.poi.util.PackageHelper.createTempFile() method as it is not used by POI and actively would re-enable the race condition vulnerability.