Add support for Retina-resolution displays

This changes `renderPlot` to send high-res images when supported by the client browser.

Remaining issues / things to do:

* Wrap `imageUrl` from `plotOutput` into an object that includes output pixel width and height.
* Receive pixel width and height in shiny.js and set `img.width` and `img.height`.
* Does the resize detection in shiny.js need to be wrapped in a function and debounced?
* Send new pixel ratio on window move (not just resize) - might not be possible without polling? See http://stackoverflow.com/questions/4319487/detecting-if-the-browser-window-is-moved-with-javascript
* Invalidate relevant observers - might not be possible?