Add curl extension loaded test

Curl.class.php

@@ -3,6 +3,10 @@ class Curl {
     const USER_AGENT = 'PHP-Curl-Class/1.0 (+https://github.com/php-curl-class/php-curl-class)';
 
     function __construct() {
+        if (!extension_loaded('curl')) {
+            throw new ErrorException('cURL library is not loaded');
+        }
+
         $this->curl = curl_init();
         $this->setUserAgent(self::USER_AGENT);
         $this->setopt(CURLOPT_RETURNTRANSFER, TRUE);

tests/run.php

@@ -24,6 +24,10 @@ class Test {
 }
 
 class CurlTest extends PHPUnit_Framework_TestCase {
+    public function testExtensionLoaded() {
+        $this->assertTrue(extension_loaded('curl'));
+    }
+
     public function testUserAgent() {
         $test = new Test();
         $test->curl->setUserAgent(Curl::USER_AGENT);