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/TokenTest.php

@@ -1,7 +1,7 @@
 <?php
 
-use JacobKiers\OAuth\Token;
-use JacobKiers\OAuth\NullToken;
+use JacobKiers\OAuth\Token\Token;
+use JacobKiers\OAuth\Token\NullToken;
 use JacobKiers\OAuth\Util;
 
 class TokenTest extends PHPUnit_Framework_TestCase