Improve documentation and build; add linting and example

-   Convert `README` to Markdown.
-   Add `Makefile`:
    -   `make build`: Build all files.
    -   `make lint`: Lint files using [JSHint].
-   Add `package.json` for distribution through [npm].
-   Improve linting.
-   Use standard `requestAnimationFrame` if available.
-   Add example for a [zoomable image][example-zoomable-image].


[example-zoomable-image]: ./examples/zoomable-image.html
[jshint]: http://www.jshint.com/
[npm]: https://www.npmjs.org/