Added ``$source``, cleaned up untar

- Added ``$source`` parameter so we can supply a drupal tarball which
lives outside the drupal module. This enables us to quickly upgrade
drupal, even before the new version is added to the files directory of
the module
- Added --no-same-owner option for tar to prevent setting ownership of
the untarred files to a non-existing user
- Added rm command to clean up after untarring