add error_log handler

[error_log](http://www.php.net/manual/en/function.error-log.php) handler
to send output to webserver SAPI handler (directly or indirectly)

src/Monolog/Handler/ErrorLogHandler.php

@@ -0,0 +1,46 @@
+<?php
+
+/*
+ * This file is part of the Monolog package.
+ *
+ * (c) Jordi Boggiano <j.boggiano@seld.be>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Monolog\Handler;
+
+use Monolog\Logger;
+
+/**
+ * Stores to PHP error_log() handler.
+ *
+ * @author Elan Ruusamäe <glen@delfi.ee>
+ */
+class ErrorLogHandler extends AbstractProcessingHandler
+{
+    protected $messageType;
+
+    /**
+     * @param integer $messageType  Says where the error should go.
+     * @param integer $level  The minimum logging level at which this handler will be triggered
+     * @param Boolean $bubble Whether the messages that are handled can bubble up the stack or not
+     */
+    public function __construct($messageType = 0, $level = Logger::DEBUG, $bubble = true)
+    {
+        parent::__construct($level, $bubble);
+        if (!in_array($messageType, array(0, 4))) {
+            throw new \InvalidArgumentException('Only message types 0 and 4 are supported');
+        }
+        $this->messageType = $messageType;
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    protected function write(array $record)
+    {
+        error_log((string) $record['formatted'], $this->messageType);
+    }
+}