Update package to support Guzzle 6 and stop supporting old versions.

examples/basic_request_example.php

@@ -0,0 +1,19 @@
+<?php
+
+use GuzzleHttp\Client;
+use HttpSignatures\Context;
+use HttpSignatures\GuzzleHttpSignatures;
+
+require __DIR__ . "/../vendor/autoload.php";
+
+$context = new Context([
+    'keys' => ['examplekey' => 'secret-key-here'],
+    'algorithm' => 'hmac-sha256',
+    'headers' => ['(request-target)', 'date'],
+]);
+
+$handlerStack = GuzzleHttpSignatures::defaultHandlerFromContext($context);
+$client = new Client(['handler' => $handlerStack]);
+
+// The below will now send a signed request to: http://example.org/path?query=123
+$response = $client->get("http://www.example.com/path?query=123", ['headers' => ['date' => 'today']]);