Automatically test for NEON instructions when building python extensions and enable them if available.

Forgot about --yes USE_NEON_INSTRUCTIONS when removing --yes a moment
ago.  It's on by default now so now --yes really is unneeded.  The dlib
policy on this kind of thing going forward is to always have a cmake
script that tests if a feature like this is available and to enable it
automatically when building python extensions.

Removed --yes option from setup.py since it has long been a noop and its presence just confuses users.

* Add python api that generates desciptor(s) from the aligned image(s)

* Remove asserts from face_recognition.py example/tutorial

* In batch_compute_face_descriptors_from_aligned_images, use for-in loop to simplify the code

Improvde the document on binding methods and the error message if the aligned image is not of size 150x150

Fixed pytest not working with python 2.7.  So turning travis tests back on for that version of python.

incompatibility between python 2.7 and pytest, but only on travis.

Removing the install_requires cmake from setup.py because the pip cmake package is hopelessly busted at the moment.

* Exposing chinese_whispers directly

* Addressing comments

* Changed description
* Added support for distance weights
* Added tests

* Improving test to check returned results

fixed check for excessive detections in loss_mmod_

Ran into the problem where dets.size() was equal to max_num_initial_dets which then throws a subscript out of range error when accesing: dets[max_num_initial_dets].detection_confidence.  This fixes that issue.

Remove Python 3.5 tests on AppVeyor because something is wrong with python 3.5 on appveyor and the other tests are sufficient for python coverage.