Bigrams and Trigrams

Features creation using has bigrams, has trigrams, count bigrams and
count trigrams
Minor changes

Added some comments
Bigrams and Trigrams Working

Final version for bi and trigrams. I'm still working on collocations.
Fixed some errors using set for comparisons
Merging

Starting merging both branches
Tf-Idf feature function

I created the tf-idf feature function, but i didn't test it. There are
some minor fix and some part of code that need to be reviewed and the
collocations function is still missing