Added target get-orig-source in debian/rules

This allows an automated repackaging a source tarball provided by upstream
by simply running # debian/rules get-orig-source.
Added debian/README.source

This files documents what changes have done to the tarball provided
by upstream and why they those have been done.
Updated debian/copyright

There is a file not covered by the license of twig, since it originates from
work done by Zend.