Add support for Faiss

[benfred]

This adds basic support for faiss on the cpu as suggested in #34. The timing results on my system seem to be on par with Annoy on the glove dataset:

[erikbern]

nice! reminds me i really want to dockerize the tests

[benfred]

I think that last commit there actually works on travis (was expecting ubuntu v16 originally, and was running on v14).

Unfortunately there seems to be a bunch of other errors unrelated to this change that will cause the build to be marked as failed. I've fixed the easy ones here benfred@7bf2397 and benfred@f6bb17b . However there still seems to be errors on falconn/nearpy.

Also, I wrote some new python bindings for nmslib a while ago, and updated the ann-benchmarks to use them here: benfred@1aec17c .

If you're interested I can submit PR's for all of these.

[erikbern]

seems fine. i'll merge this for now, will revisit the travis integration and also dockerization

[erikbern]

thanks!