Implement Blog model and controllers.

Until Directories and Features are implemented, using a singleton Blog resource for simplicity at /blog.

- BlogController handles public actions, currently only #show.
- Admin::BlogController handles all RESTful actions, requires an authenticated user.

Additional notes:

- Added Admin::AdminController as base for all admin controllers, and refactored Admin::SettingsController to inherit from it.
- Added breadcrumbs partial.
- Added spec/support/examples for shared examples.