Basic auth decorator

This change adds a new `.basic_auth_required()` decorator to the LDAP class, with a new configuration field to specify the Basic authentication realm to use. It also updates the documentation for `.login_required()` and `.group_required()` to emphasise how they differ from `.basic_auth_required()`, and adds an example demonstrating `.basic_auth_required()`.

I agree that HTTP basic authentication isn't the prettiest thing in the world, but it sure makes it easy to get started and verify your LDAP server settings are correct.

Fixes #7.