use redirect_to_login instead of HttpResponseRedirect in get_403_or_None

- using redirect_to_login facilitates correct interpretation of
settings.LOGIN_URL when it is a class instead of path
- matches functionality in django-braces
add tests for login url that is a class instead of url
skip testing login_url set to class when django version < 1.5
use override_settings instead of directly modifying login_url
switch to assertRedirects instead of assertTrue

- ignores test for redirect with class based LOGIN_URL for django
versions < 1.5
Merge pull request #225 from wvolz/permredirectfix

use redirect_to_login instead of HttpResponseRedirect in get_403_or_None