Add initial support for SOURCE_DATE_EPOCH

Add initial support for SOURCE_DATE_EPOCH

This allows to create reproducible binary files without creation/modification-timestamp
being set when environment variable SOURCE_DATE_EPOCH is set.

See https://reproducible-builds.org/docs/source-date-epoch/ for the related specification.

For now, we ensure that Zip-file entries set the modification time to 1970-01-01,
which seems to be enough to make simple OOXML files reproducible.

There are likely some other places where resulting files are not reproducible, some
more testing will be necessary to identify other areas that should take this into
account as well.

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1919236 13f79535-47bb-0310-9956-ffa450edef68