py: make it possible to specialise web element

This patch makes it possible for individual drivers to
provide specialised WebElement classes through overriding
selenium.webdriver.remote.webelement.WebElement
and defining the _web_element_cls property on
selenium.webdriver.remote.webdriver.WebDriver.

This would enable client bindings for a particular browser to provide
custom methods also on the web element level.  Currently vendor extension
commands are only possible on the WebDriver level.

py/selenium/webdriver/remote/webdriver.py

@@ -53,6 +53,8 @@ class WebDriver(object):
      - error_handler - errorhandler.ErrorHandler object used to handle errors.
     """
 
+    _web_element_cls = WebElement
+
     def __init__(self, command_executor='http://127.0.0.1:4444/wd/hub',
                  desired_capabilities=None, browser_profile=None, proxy=None,
                  keep_alive=False, file_detector=None):
@@ -187,18 +189,16 @@ def _wrap_value(self, value):
             for key, val in value.items():
                 converted[key] = self._wrap_value(val)
             return converted
-        elif isinstance(value, WebElement):
+        elif isinstance(value, self._web_element_cls):
             return {'ELEMENT': value.id, 'element-6066-11e4-a52e-4f735466cecf': value.id}
         elif isinstance(value, list):
             return list(self._wrap_value(item) for item in value)
         else:
             return value
 
     def create_web_element(self, element_id):
-        """
-        Creates a web element with the specified element_id.
-        """
-        return WebElement(self, element_id, w3c=self.w3c)
+        """Creates a web element with the specified `element_id`."""
+        return self._web_element_cls(self, element_id, w3c=self.w3c)
 
     def _unwrap_value(self, value):
         if isinstance(value, dict) and ('ELEMENT' in value or 'element-6066-11e4-a52e-4f735466cecf' in value):