Added RequestInterface to make integration easier.

Since now only a RequestInterface is needed instead of a Request object,
it becomes easier to use the Server component with a different Request
object, such as the Symfony or Zend Framework Request objects.

This will now only need a small wrapper, instead of extending and
rewriting the existing Request object.

tests/HmacSha1Test.php

@@ -57,7 +57,7 @@ private function getSignatureMethod()
     private function getRequest()
     {
         return m::mock('JacobKiers\OAuth\Request', function ($mock) {
-            $mock->shouldReceive('getSignatureBaseString')
+            $mock->shouldReceive('getOAuthSignatureBaseString')
                 ->withNoArgs()
                 ->andReturn('POST&http%3A%2F%2Fexample.com%2Ffoobar&oauth_signature_method%3DHMAC-SHA1')->once();
         });