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/RequestTest.php

@@ -1,7 +1,7 @@
 <?php
 
 use Mockery as m;
-use JacobKiers\OAuth\Request;
+use JacobKiers\OAuth\Request\Request;
 
 class RequestTest extends PHPUnit_Framework_TestCase
 {