Make the Client parameter optional for requests

Make the Client parameter optional when creating auth token, customer or
order requests, and create a new Client instance within the abstract
request if one isn't passed in.

tests/Glassboxx/Request/AuthTokenRequestTest.php

@@ -22,8 +22,7 @@ class AuthTokenRequestTest extends TestCase
             ->method('request')
             ->with(
                 'POST',
-                AuthTokenRequest::BASE_URL
-                .AuthTokenRequest::ENDPOINT,
+                AuthTokenRequest::ENDPOINT,
                 [
                     'query' => [
                         'password' => 'secret',