Support for raw chinese_whispers

This commit introduces chinese_whispers method without any "helpers" on
PHP side. Users needs to take care of everything (building edges from
128D face chip vector, for example), but this is exposed for people
that need low-level call and want to calculate distances and build edges
in PHP directly.

tests/chinese_whispers_edge_not_2_element_error.phpt

@@ -0,0 +1,20 @@
+--TEST--
+Edge given in edges array for chinese_whispers functions is not having all values to be arrays with 2 elements
+--SKIPIF--
+<?php if (!extension_loaded("pdlib")) print "skip"; ?>
+--FILE--
+<?php
+try {
+    dlib_chinese_whispers([[0,0], [1]]);
+} catch (Exception $e) {
+    var_dump($e->getMessage());
+}
+try {
+    dlib_chinese_whispers([[0,0], [1,1,1]]);
+} catch (Exception $e) {
+    var_dump($e->getMessage());
+}
+?>
+--EXPECT--
+string(42) "Edges need to contain exactly two elements"
+string(42) "Edges need to contain exactly two elements"