ExtendedURLValidator does not pass down verify_exist in it's __init__
method. But as it's super class URLValidator then sets self.verify_exists to
False, any passed value was ignored.

This is required to allow the pages component of the dashboard to
continue to work correctly.

* Tidy implementation of validator
* Tests improved and moved to tests/integration (as they call resolve
  internally)

model from within forms.validators

I added a new validator to oscar.core.validators that validates
absolute and relative URL and checks if the URL already exists. We
use this functionality when creating flatpages in the dashboard to
check if the URL can be used.
The validation includes *two* calls to ExtendedURLValidator to
validate the URL in the first run and then check if it already
exists in the second. This might not be the most efficient way
of handling it. The reason to do it that way is that it reduces
the amount of code that has to be replicated.

There is now a model field, a form field and a validator.