[MRG] Locality Sensitive Hashing for approximate nearest neighbor search(GSoC)

I would cut the remainder of this section and move it at the end of the chapter on ANN under a new section called "Mathematical description of LSH". The rational is to reorganize that documentation to help users solve their problem first with practical usage info (name of the class, how to use it and what to use it for, impact of the hyperparameters) and then with more theoretical information that help get a deep understanding on how it works internally.

For instance have a look at the organization of the chapter on linear models trained with the SGD algorithm: http://scikit-learn.org/stable/modules/sgd.html