Collapse "min" into project root and get unit tests working.

Fixes #472

lib/Minify/SourceSet.php

@@ -0,0 +1,29 @@
+<?php
+/**
+ * Class Minify_SourceSet
+ * @package Minify
+ */
+
+/**
+ * @package Minify
+ */
+class Minify_SourceSet {
+
+    /**
+     * Get unique string for a set of sources
+     *
+     * @param Minify_SourceInterface[] $sources Minify_Source instances
+     *
+     * @return string
+     */
+    public static function getDigest($sources)
+    {
+        $info = array();
+        foreach ($sources as $source) {
+            $info[] = array(
+                $source->getId(), $source->getMinifier(), $source->getMinifierOptions()
+            );
+        }
+        return md5(serialize($info));
+    }
+}