Added PsrHandler

This handler allows applications introducing Monolog to continue using
their existing PSR-3 compliant logger(s) in conjunction with the various
Monolog handlers available.

tests/Monolog/Handler/PsrHandlerTest.php

@@ -0,0 +1,50 @@
+<?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\TestCase;
+use Monolog\Logger;
+
+/**
+ * @covers Monolog\Handler\PsrHandler::handle
+ */
+class PsrHandlerTest extends TestCase
+{
+    public function logLevelProvider()
+    {
+        $levels = [];
+        $monologLogger = new Logger('', [], []);
+
+        foreach ($monologLogger->getLevels() as $levelName => $level) {
+            $levels[] = [$levelName, $level];
+        }
+
+        return $levels;
+    }
+
+    /**
+     * @dataProvider logLevelProvider
+     */
+    public function testHandlesAllLevels($levelName, $level)
+    {
+        $message = 'Hello, world!';
+        $context = ['foo' => 'bar'];
+
+        $psrLogger = $this->getMock('Psr\Log\NullLogger');
+        $psrLogger->expects($this->once())
+            ->method(strtolower($levelName))
+            ->with($message, $context);
+
+        $handler = new PsrHandler($psrLogger);
+        $handler->handle(['level' => $level, 'level_name' => $levelName, 'message' => $message, 'context' => $context]);
+    }
+}