Allow compound extensions (.tar.gz, .tar.xz, ...)

This fix is not complete, as we should also change frontend/pages/admin_course.py:364 to generate the filename in the downloaded archive.
Clean all previously running jobs on restart
Fix hard jobs
Add new hooks, move a part of PluginManager to the backend (to allow to hook the backend), increase the quantity of information available in the backend, allow plugins to use templates
Some documentation about hooks
Fix documentation about installation on OS X. Simpleldap has some problems. This part of the documentation needs a refactor...
Forgot to add the new HookManager
Code review is a must-have
Fix detection of some compound file extensions. See PR #19.
Merge pull request #19 from layus/patch-1

Allow compound extensions (.tar.gz, .tar.xz, ...)