Fixed #800 -- Fixed bug in treatement of underscores and percent signs in SQLite backend. In order to do this, changed OPERATOR_MAPPING in DB backends to include a format string of the field value, because ESCAPE comes after the field value.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@1326 bcc190cf-cafb-0310-a4f2-bffc1f526a37