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

`query` is also `dtype='>u4'`. `_convert_to_hash` method returns the hashed query in that data type.

I think this method with binary search is faster than having sequential searches on `bit_string_array`. If `n` is the index size, approximately it'll take `32*n` time in worst case which is O(n). With this method, there is only `log(32)` searches for hash length and each of them take `2*log(n)` which is O(logn).