Downloadable zip file for shell filesystem.

This PR is a proposal to add a downloadable zipfile containing the example filesystem used in the Unix shell lessons. This is to help take advantage of @rbeagrie 's recent fixes which make the example filesystem consistent with the lessons. Having this filesystem available as a zip file allows students to get it without needing to install or learn git first. Usually students cover the shell lessons before the git lessons.

I think that adding the zipfile itself is relatively uncontroversial. However I've also edited the first shell lesson to provide some instructions to students to download and unzip the zipfile, and I'm less sure that this is an appropriate thing to do, or that I've done it in the best way. Comments welcome!

The added files are

    novice/shell/filesytem.zip
    novice/shell/filesystem.tar.gz

although only `filesystem.zip` is referenced in the lesson text. The `.tar.gz` file is there so that it's available for instructors in case of problems.

The download-and-unzip instructions are necessarily slightly fuzzy since the location of a downloaded file will depend on what system you're on. So, for those workshops where instructors want to have students do this, I expect that some students will need help. My feeling though is that it's a worthwhile thing to be able to do.