Add a localurls.py construct similar to localsettings.py

Currently, to use addon applications like `django-browserid`, one has to add some paths to the URLconf. Since it's undesirable to modify the core code just to add an addon, I propose creating a localurls.py file that enables the extension of the URLconf like localsettings.py enables the extension of the core settings.

Let me know if there are any glaring issues with this or if this is a change that I should ping the development mailing list about.