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

Actually a linear space seems to be more informative to visualize the curve:

``python
n_samples_values = np.linspace(1e3, 1e6, 5).astype(np.int)
```

When I made this change (and removed the call to `plt.semilogx`), it seems that LSH Forest query time increases approximately linearly. I would have expected a sub-linear query time progression. Is that expected?