Add name length check to the HipChatHandler

As explained in its docblock, the validation function will fall back to
a simple strlen check if mb_strlen is not available. However, that very
specific case is not one that was deemed important to support, as anyone
using UTF-8 in their code, without having mbstring available will
probably have bigger problems to worry about.

Fixes #289

tests/Monolog/Handler/HipChatHandlerTest.php

@@ -156,4 +156,12 @@ class HipChatHandlerTest extends TestCase
 
         $this->handler->setFormatter($this->getIdentityFormatter());
     }
+
+    /**
+     * @expectedException InvalidArgumentException
+     */
+    public function testCreateWithTooLongName()
+    {
+        $hipChatHandler = new \Monolog\Handler\HipChatHandler('token', 'room', 'SixteenCharsHere');
+    }
 }