Update snipmate's supertab handling for the latest supertab version.

I made some changes to supertab to deal with users who have mapped <c-n>/<c-p> to something else and didn't want supertab clobbering those mappings. So now supertab has a couple <Plug> mappings for forward/backward completion and some code to retain the user's original mappings. However, snipmate is still relying on <c-n>/<c-p> to kick off supertab, so I made a patch to utilize the new <Plug> mappings when available (should retain backwards compatibility with older supertab versions), which resolves any issues I've encountered with supertab + snipmate + user mapped <c-n>/<c-p>. Let me know if you have any questions regarding these changes. Thanks!