Minify now allows static file serving

With slightly altered URLs, Minify can cache files so they're served directly
from the filesystem instead of through PHP. A simple library helps create URLs
and clearing the cache.

See `static/README.md` for details.

static/lib.php

@@ -0,0 +1,68 @@
+<?php
+
+namespace Minify\StaticService;
+
+/**
+ * Build a URI for the static cache
+ *
+ * @param string $static_uri E.g. "/min/static"
+ * @param string $query E.g. "b=scripts&f=1.js,2.js"
+ * @param string $type "css" or "js"
+ * @return string
+ */
+function build_uri($static_uri, $query, $type) {
+    $static_uri = rtrim($static_uri, '/');
+    $query = ltrim($query, '?');
+
+    $ext = ".$type";
+    if (substr($query, - strlen($ext)) !== $ext) {
+        $query .= "&z=$ext";
+    }
+
+    $cache_time = get_cache_time();
+
+    return "$static_uri/$cache_time/$query";
+}
+
+/**
+ * Get the name of the current cache directory within static/. E.g. "1467089473"
+ *
+ * @param bool $auto_create Automatically create the directory if missing?
+ * @return null|string null if missing or can't create
+ */
+function get_cache_time($auto_create = true) {
+    foreach (scandir(__DIR__) as $entry) {
+        if (ctype_digit($entry)) {
+            return $entry;
+            break;
+        }
+    }
+
+    if (!$auto_create) {
+        return null;
+    }
+
+    $time = (string)time();
+    if (!mkdir(__DIR__ . "/$time")) {
+        return null;
+    }
+
+    return $time;
+}
+
+function flush_cache() {
+    $time = get_cache_time(false);
+    if ($time) {
+        remove_tree(__DIR__ . "/$time");
+    }
+}
+
+function remove_tree($dir) {
+    $files = array_diff(scandir($dir), array('.', '..'));
+
+    foreach ($files as $file) {
+        is_dir("$dir/$file") ? remove_tree("$dir/$file") : unlink("$dir/$file");
+    }
+
+    return rmdir($dir);
+}