Expand docstrings for .login_required() and .group_required()

Now .login_required() and .group_required() document what they require
LDAP_LOGIN_VIEW to do.
Add the .basic_auth_required() decorator.

This is much less flexible than .login_required() (because HTTP
authentication is less flexible than custom login pages), but easier to
use in a simple proof-of-concept.
Add an example of using .basic_auth_required()
Merge pull request #8 from OptiverTimAll/basic-auth-decorator

Basic auth decorator