Fix issue #10

[guilhermenoronha]

I figured out the problem. When the sentence has all token as "O" the _tokens method creates begins with one extra position causing the index out of range in ts[idx]. To fix that I checked the size of the array before comparing. If the size of begins is less than ts size, then there is at least one token that isn't "O". So the method can add the "O" token in the last index of the array.