Define URL generator when instantiating Page #2

This makes it possible to specify a function to generate the URL for a
page during instantiation instead of from the template.

When used in a Pyramid project, this could serve as an example:

    def url_maker(page_number):
        return request.current_route_url(_query=dict(page=page_number))

Fixes https://github.com/Signum/paginate/issues/2