Backport twig support to D7.

This does not modify any files it only adds TDF and Twig libraries
and the twig.engine file in the core directory next to phptal
Issue #1934568 by David_Rothstein, pwolanin: Allow sites using the 'image_allow_insecure_derivatives' variable to have partial protection from the security issues fixed in Drupal 7.20.
Prepare Drupal 7.21 release.
Merge tag 'tags/7.21' into drupal-7.21

7.21 release
Merge branch 'drupal-7.21'
Update to Drupal 7.21 - maintenance release focusing on users impacted by the image derivative changes in 7.20. For more information see:

http://drupal.org/drupal-7.21
Clean up TFD function and file names.

The function and file names were out of sync and failing on a posix
complaint file system
Merge branch 'master' of git://github.com/pantheon-systems/drops-7
Merge pull request #4 from micheas/master

upgrade to 7.21 and add twig support