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.

src/JacobKiers/OAuth/Consumer/ConsumerInterface.php

@@ -0,0 +1,48 @@
+<?php
+/**
+ * OAuth
+ *
+ * @package OAuth
+ * @author Jacob Kiers <jacob@alphacomm.nl>
+ * @license https://raw.github.com/jacobkiers/OAuth/master/LICENSE MIT
+ * @link https://github.com/jacobkiers/OAuth
+ */
+
+namespace JacobKiers\OAuth\Consumer;
+
+/**
+ * Consumer holds the properties of a single Consumer / consumer.
+ *
+ * @package OAuth
+ * @author Jacob Kiers <jacob@alphacomm.nl>
+ */
+interface ConsumerInterface
+{
+    /**
+     * Get the callback URL.
+     *
+     * @return string
+     */
+    public function getCallbackUrl();
+
+    /**
+     * Set the callbackURL
+     *
+     * @param string $callback_url
+     */
+    public function setCallbackUrl($callback_url);
+
+    /**
+     * Return the Consumer key.
+     *
+     * @return string Consumer key.
+     */
+    public function getKey();
+
+    /**
+     * Return the Consumer secret
+     *
+     * @return string
+     */
+    public function getSecret();
+}