Adds a "Principal" model.

Includes a `token` attribute to be used during authentication.
Adds `factory_girl_rails` for model factories.
Introduces a simple error page.
Generates the token when a Principal is created.
Introduces token auth in a before action hook.

`before_action :authenticate` expected to be overridden for controllers needing finer-grained control over authentication.
Excludes certain pages from authentication.

Specifically the error, contact, pre-qualification and identification pages.
Adds page objects for home and error pages.
Adds a bare-bones factory for the "Principal" model.
Simple feature spec for token authentication.