Restore logging for cache operations

`AbstractCache` exposes the ability to set a logger. Default behavior
automatically passes the logger reference down to the cache, unless a
specific cache instance was provided.

src/Mustache/Cache/AbstractCache.php

@@ -0,0 +1,27 @@
+<?php
+
+abstract class Mustache_Cache_AbstractCache implements Mustache_Cache
+{
+    private $logger = null;
+
+    public function getLogger()
+    {
+        return $this->logger;
+    }
+
+    public function setLogger($logger = null)
+    {
+        if ($logger !== null && !($logger instanceof Mustache_Logger || is_a($logger, 'Psr\\Log\\LoggerInterface'))) {
+            throw new Mustache_Exception_InvalidArgumentException('Expected an instance of Mustache_Logger or Psr\\Log\\LoggerInterface.');
+        }
+
+        $this->logger = $logger;
+    }
+
+    protected function log($level, $message, array $context = array())
+    {
+        if (isset($this->logger)) {
+            $this->logger->log($level, $message, $context);
+        }
+    }
+}