Add authorize checks for resources

[quorak]

• Authorization: Added per-resource authorize blocks and an authorized? check; access enforced for resources/read (Fixes Add authorize checks for resources #106).
• Header injection: Request headers are passed into resource instances for read operations (resource.rb, server.rb).
• Documentation: Updated CHANGELOG.md, README.md and resources.md with examples and guidance for resource authorization and header usage.
• Tests: Added/updated specs in resource_spec.rb and server_spec.rb covering authorization, templated resources, and header injection.

Fixes #106