Add 'log' command to support logging API

[ildar-aim]

The patch is

--- a/php-webdriver/PHPWebDriver/WebDriverSession.php
+++ b/php-webdriver/PHPWebDriver/WebDriverSession.php
@@ -46,6 +46,7 @@ class PHPWebDriver_WebDriverSession extends PHPWebDriver_WebDriverContainer {
       'doubleclick' => 'POST',
       'location' => array('GET', 'POST'),
       'file' => 'POST',
+      'log' => 'POST',
     );
   }

The usage is
https://github.com/Element-34/php-webdriver/wiki/How-to-check-javascript-errors