Refactor from/toString methods to codecs

tests/Codec/StringCodecTest.php

@@ -0,0 +1,18 @@
+<?php
+
+namespace Rhumsaa\Uuid\Codec;
+
+use Rhumsaa\Uuid\TestCase;
+
+class StringCodecTest extends TestCase
+{
+    public function testDecodeParsesUuidStringCorrectly()
+    {
+        $codec = new StringCodec();
+
+        $uuid = $codec->decode('ff6f8cb0-c57d-11e1-9b21-0800200c9a66');
+
+        $this->assertInstanceOf('\Rhumsaa\Uuid\Uuid', $uuid);
+        $this->assertEquals('ff6f8cb0-c57d-11e1-9b21-0800200c9a66', $uuid->toString());
+    }
+}