Make sure that eggs and public dirs permissions are OK for Linux

As for now on a Linux system the pdf/ and tempfiles/ directories in crdppf/static/public are not writeable by the apache user (create an error when generating a PDF extract). I have also noticed permissions issues with the httplib2 egg.

This PR suggests to use a "fix-perm" system inspired by what is done in c2cgeoportal:
https://github.com/camptocamp/c2cgeoportal/blob/master/c2cgeoportal/scaffolds/update/CONST_buildout.cfg_tmpl#L103-L115
The commands are actually only executed for Linux systems.