Add IFrenquency.merge(IFrequency) 

Unlike ICardinality, nor IFrenquency nor CountMinSketch currently define a merge function. According to [Approximating Data with the Count-Min Data Structure](http://dimacs.rutgers.edu/~graham/pubs/papers/cmsoft.pdf) 

> But more than this, one can build sketches of different subsets of the data (after agreeing on the same parameters w, d and set of hash functions to use), and these sketches can be combined to give the sketch of the union of the data. Sketch combination is straightforward: given sketch arrays of size w × d, they are combined by summing them up, entry-wise.

Is there a specific reason for this function to be absent ? Since I need this feature for one of my project, I will happily implement it and send a pull request if you think that it would be an useful addition.