Addig activity loading via Git URLs

Instead of manually having to fetch the activities beforehand, the
system supports sending a Git URL and checking out the activities all by
itself.

This behavour can be tested by running the following command:

    curl -d'ref=master' \
        -d'repository=https://github.com/kgilden/knpu-course-te' \
        -d'activity=example' -d 'output-format=json' -d 'file[index.php]=<?php
        echo 'foo';' http://example-runner.com/app_dev.php/check