Reworking :requires_authentication

Instead of the two parameters for the authentication layer, I'm
prefering the singular configuration of :authentication_layer.

It is possible that I should extract the authentication building into
an AuthenticationLayer.build method.
Tightening up runner expectations regarding auth
Merge pull request #27 from ndlib/reworking-authentication_layer-configuration

Reworking authentication layer configuration
Tidying up coordination of authorization layer

Instead of relying on two external flags for authorization
configuration, I'm preferring a singular flag. It is likely that I
should extract an AuthorizationLayer.build method.

There is also a bit of tidying up of specs to better convey intent and
reasoning.
Merge pull request #28 from ndlib/reworking-authorization-layer-configuration

Tidying up coordination of authorization layer