Add permutation module

Hi,
I've add a new module permutation, which aims to find the neighbour bucket keys. 
I've add two new methods in engine.py:
1) build_permuted_index(), which build a permutedIndex for each binary hashes.
2) neighbours_p(), which is similar to neighbours, except it find the neighbour keys of the matching bucket keys.
I've write a example in nearpy.tests.permutation_tests. It will compare the neighbours returned by neighbours(), neighbours_p() and the real nearest neighbours.

If you have any questions or coding format issues, feel free to contact me at github or xingshi@usc.edu