Fixed #4131: added an "escapejs" filter for use in JavaScript strings, and...

Fixed #4131: added an "escapejs" filter for use in JavaScript strings, and updated the documentation on addslashes to point to the new ticket. Featuring contributions from Ned Batchelder, Jeremy Dunck, and Andy Durdin.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@6892 bcc190cf-cafb-0310-a4f2-bffc1f526a37