Make sure ZIP is installed

[cvonkleist]

There is an error when exporting if the zip command isn't installed on the host. See 3153fd0.

Suggested actions to fix:

1. Check to see if which zip returns an exit status of 0 before this line:

   archon/packages/creators/db/export-eac.inc.php

   Line 159 in 3153fd0

   exec("zip -r $tmp_zip $foldername");

2. Update documentation to say that zip is a system dependency