Extract cache interface

Refactoring to allow for alternative cache implementations. Includes
implementations for the default noop (i.e. un-cached) behavior and a
filesystem-based cache whose configuration is consistent with the
existing Mustache_Engine constructor.

Includes updated and new tests.

test/Mustache/Test/Cache/FilesystemCacheTest.php

@@ -0,0 +1,58 @@
+<?php
+
+/**
+ * @group functional
+ */
+class Mustache_Test_Cache_FilesystemCacheTest extends PHPUnit_Framework_TestCase
+{
+    private static $tempDir;
+
+    public static function setUpBeforeClass()
+    {
+        self::$tempDir = sys_get_temp_dir() . '/mustache_test';
+        if (file_exists(self::$tempDir)) {
+            self::rmdir(self::$tempDir);
+        }
+    }
+
+    public function testCacheGetNone()
+    {
+        $key = 'some key';
+        $cache = new Mustache_Cache_FilesystemCache(self::$tempDir);;
+        $cached = $cache->get($key);
+
+        $this->assertNull($cached);
+    }
+
+    public function testCachePut()
+    {
+        $key = 'some key';
+        $value = 'some value';
+        $cache = new Mustache_Cache_FilesystemCache(self::$tempDir);;
+        $cache->put($key, $value);
+        $cached = $cache->get($key);
+
+        $this->assertEquals($cached, $value);
+    }
+
+    private static function rmdir($path)
+    {
+        $path = rtrim($path, '/').'/';
+        $handle = opendir($path);
+        while (($file = readdir($handle)) !== false) {
+            if ($file == '.' || $file == '..') {
+                continue;
+            }
+
+            $fullpath = $path.$file;
+            if (is_dir($fullpath)) {
+                self::rmdir($fullpath);
+            } else {
+                unlink($fullpath);
+            }
+        }
+
+        closedir($handle);
+        rmdir($path);
+    }
+}