Approx min sketch

First try of CMSketch. Still many things to fix:

1) Parameter settings , not clear way to do it or based on my test good defaults.
2) It is too "Slow"  compared to the big memory version. Probably some optimization is necessary. 
3) Not a clear way to compute the score for bigrams for the phrases detection part. The score calculation currently uses the length of the vocab but CMSketch has no length nor an approximate. 
4) More testing is necessary.