fix supertab <cr> map chaining logic

Looks like the logic for deciding if a cr should be issued by
SelectCompletion has been wrong. Negating the previous logic seems to be
what I had intended (unless there was some other edge case I'm
forgetting).

refs #138
disable g:SuperTabCrMapping by default

This one feature causes more headaches than any other, with very little
benefit to show for it.