I think sparate 'can not' is better.
Reimplement normalize_tokens, use a tmp array to make it more clear, by the way, make a strict array in rules.
fix casesensitive bug to pass ci, think about 'cannot' or 'can not'?
Merge branch 'master' of https://github.com/liwenzhu/natural into liwenzhu-master
changed the spec to expect 'can not' instead of 'cannot'
Merge branch 'master' of https://github.com/NaturalNode/natural