Face recognition

This change adds support to retrieve 128D face descriptor for a given
landmark. Since now we have full pipeline, README.md has "general usage"
section and integration test is added. Also, return from
FaceLandmarkDetection is changed, so it can be given to FaceRecognition
without changes. All obtained values are crosschecked to match with values
from python versions (however, if num_jitters is > 1 in FaceRecognition,
values don't match between PHP and Python, I suspect it is related to usage
of dlib::rand, but still investigating)..

tests/face_recognition_ctor_error.phpt

@@ -0,0 +1,15 @@
+--TEST--
+Testing FaceRecognition constructor without arguments
+--SKIPIF--
+<?php if (!extension_loaded("pdlib")) print "skip"; ?>
+--FILE--
+<?php
+try {
+	new FaceRecognition();
+} catch (Exception $e) {
+	var_dump($e->getMessage());
+}
+?>
+--EXPECT--
+Warning: FaceRecognition::__construct() expects exactly 1 parameter, 0 given in /home/branko/pdlib/tests/face_recognition_ctor_error.php on line 3
+string(43) "Unable to parse face_recognition_model_path"