add merge tune to extract tunes by title

update Tune::merge() and add Tune::equals()

tests/unit/AbcParser/Domain/Core/TuneTest.php

@@ -212,7 +212,6 @@ class TuneTest extends \PHPUnit_Framework_TestCase
 
         $result = $targetTune->merge($sourceTune);
 
-        $this->assertEquals($expected, $result);
-
+        $this->assertTrue($expected->equals($result));
     }
 }