Expose Dream.static file loaders

[aantron]

• [x] The default file loader would probably be useful to wrap in a middleware-like fashion, so that users don't have to implement the actual file reading, but can easily add custom headers, especially ETag:.
• [ ] Also expose a crunch-friendly loader, and add an example.
• [ ] Simplify static loader and move it to Responses section.

See Dream.static ?handler (rename to ?loader?).