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

This is an important parameter, and applies universally unlike `n_candidates`, `n_neighbors`, `radius` and `radius_cutoff_ratio` each of which is only used in one of the two query methods. Please make this the second parameter to emphasise it.

The importance of this parameter, and its unfamiliarity to users of whitebread LSH also makes me wonder whether it should be the subject of an example (although I appreciate that it may interact with other parameters in tricky ways).