Add Browser.IsDevTools() #169

There should be a way to differentiate if a browser is a Developer Tools 
window. For example in LoadHandler.OnLoadStart() we're executing app specific 
code and when Browser.ShowDevTools() is called this will result in OnLoadStart 
being called for developer tools window as well.

In the meantime use this code to detect if this is a DevTools browser:

  if re.search(r"^http://127\.0\.0\.1:\d+/devtools/devtools\.html",
               frame.GetUrl()):