Use Interfaces anywhere.

In practice, there were still too many concrete classes, which makes
integration into a framework hard. To overcome this, the codebase has
been refactored to use Interfaces when a resource is needed.

All necessary Interfaces have been created, and the existing concrete
classes now implement these interfaces.

tests/ConsumerTest.php

@@ -0,0 +1,20 @@
+<?php
+
+use JacobKiers\OAuth\Consumer\Consumer;
+
+class ConsumerTest extends PHPUnit_Framework_TestCase
+{
+    public function testKeyAndSecretAreSet()
+    {
+        $consumer = new consumer('foo', 'bar');
+        $this->assertEquals('foo', $consumer->getKey());
+        $this->assertEquals('bar', $consumer->getSecret());
+    }
+
+    public function testCallbackUrlIsSet()
+    {
+        $consumer = new consumer('foo', 'bar', 'http://example.com/foobar');
+        $this->assertEquals('http://example.com/foobar', $consumer->getCallbackUrl());
+    }
+
+}