shiny 0.5.0
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* License for Shiny changed to GPL >= 2, and a few files are dual-licensed
  under the GPL >= 2 and BSD 3-clause.

* Switch from websockets package for handling websocket connections to httpuv.

* New method for detecting hidden output objects. Instead of checking that
  height and width are 0, it checks that the object or any ancestor in the DOM
  has style display:none.

* Add `clientData` reactive values object, which carries information about the
  client. This includes the hidden status of output objects, height/width plot
  output objects, and the URL of the browser.

* Add `parseQueryString()` function.

* Add `renderImage()` function for sending arbitrary image files to the client,
  and its counterpart, `imageOutput()`.

* Add support for high-resolution (Retina) displays.

* Fix bug #55, where `renderTable()` would throw error with an empty data frame.