Use a different node package for zipping

[adm-zip](https://github.com/cthackers/adm-zip) appears to be broken. I ran into issues extracting certain files (e.g. images larger than 100KB).

I found another library, [jszip](https://github.com/Stuk/jszip). It also does zipping and works well on the few projects I tried. One difference is that it doesn't have an API for adding the contents of an entire folder to the zip, so you have to read all the files from `config.webPackage.outPath` recursively.