[Web Apps] A few improvements of the Django application.

1. The zoom level is now set through the more standard (Google) 'z' parameter.
2. The map type is now set through the 'mt' parameter.
3. The error pages (404 and 500) are passed the request_path (failing URL),
   so that they can display it.
4. The 500 error page is now also generated, like the 404 one.
5. Improved the README documentation.
[Doc] Fixed a typo in the README file.