add helper class to analyze response.location

[bdauvergne]

Suggestion by @gawel, a new webtest.utils.URL class used to return response.location/content_location/url and simplify tests, with pytest you can write things as simple as:

     assert resp.location.match('https://example.org/a/b/c')
     assert resp.location.match('?foo=bar')

and you get clear errors on output

        def test_location(self):
        app = webtest.TestApp(debug_app)
        res = app.post('/')
        res.location = 'http://pylons.org'
        self.assertTrue(res.location.match('http://'))
>       self.assertTrue(res.url.match('https://truc.org'))
E       AssertionError: scheme differs http != https is not true