Switch to Apache POI for XLSX support. Fixes #136

[tfmorris]

Fixes #136

This is a proof of concept port to Apache POI. It has only received VERY LIMITED testing and I'm not a picture uploader, so someone else who actually uses the tool should take it over and make sure it works as expected.

It now writes .xlsx files and reads both .xls and .xlsx. Apache POI doesn't seem to support column autosize for .xls files, so I didn't make it an option to write that format.

In addition to testing, packaging should also be reviewed. It may be possible to remove some of the bundled JARs (or better yet, switch to Maven for dependency management). The current build size is 29MB up from the previous 13MB.