Fix issue #9 (negate_sequence bug)

[josmithua]

Please review my fix for issue #9.

I use the stripped version of the word (lowered and stripped of delims) to see if it equals "not", "cannot", "no", or if it ends with "n't". If any of these are true, then negation is negated.

This fixes both issues of:

1. negation being changed when seeing words containing ["no", "not", "n't"] (e.g. "know")
2. negation not being changed when seeing things that are uppercase that should trigger negation being changed but don't. (i.e., capital "DON'T")

I added the word "cannot" because if I checked if stripped.endswith("not") then that would match words like "knot" and "whatnot". I'm assuming negation is wanted when observing the word "cannot". If this is a wrong assumption please let me know.

Cheers, @Sm1th