NOJIRA Migrate from ZipFile to PharData

Zipfile produces corrupt files on CentOS with PHP5.4 and I'm not in the mood to reinvent the wheel so I thought we might as well migrate to something that works out of the box.

The phar extension is not really a new requirement, we've been using it in the zip extraction code of BaseModell::_processMedia for ages. Now it's a formal requirement with config check and everything.

I'm not sure but I think the extension is part of the core distribution since PHP5.3. It's available in the default php 5.4 packages in Debian and CentOS. No extra setup required.

I replaced all occurrences I could find but kept ZipFile around for now because it's probably still in use in Pawtucket.