Implement the pairwise similarities module.

[marcelcaraciolo]

In this task we need to implement the most used pairwise similarities provided in this source-code:

https://github.com/muricoca/crab/blob/master/scikits/crab/metrics/pairwise.py

We need check each one as also optimize them individually.

I think some of them are not optimized.

For inspiration check this:

https://github.com/scikit-learn/scikit-learn/blob/master/sklearn/metrics/pairwise.py
http://svn.apache.org/repos/asf/mahout/trunk/core/src/main/java/org/apache/mahout/cf/taste/impl/similarity/

[marcelcaraciolo]

https://github.com/paulasmuth/recommendify/blob/master/ext/jaccard.c

[marcelcaraciolo]

https://github.com/scikit-learn/scikit-learn/blob/master/sklearn/metrics/pairwise_fast.pyx