Add Coinbase spot rate example

examples/coinbase_spot_rate.php

@@ -0,0 +1,10 @@
+<?php
+require '../src/Curl.class.php';
+
+
+$curl = new Curl();
+$curl->get('https://coinbase.com/api/v1/prices/spot_rate');
+
+echo
+    'The current price of bitcoin at Coinbase is ' .
+    '$' . $curl->response->amount . ' ' . $curl->response->currency . '.' . "\n";