update snipmate to handle latest supertab version

supertab now has <Plug>SuperTabForward/<Plug>SuperTabBackward mappings
to help deal with users who have <c-n>/<c-p> mapped to something, so
update snipmate to make use of the plug mappings.
update backwards snippet as well for latest supertab
Merge branch 'master' of https://github.com/ervandew/snipmate.vim into HEAD
added new snippets
added sass snippets