Add shiny metadata channel and send pixel ratio

This adds support for retina-resolution displays
Partial change to reactiveValues
Add manageInputs() to handle metadata and normal inputs
Rename .metadata to clientData
Rename shinyapp to shinysession, and .shinyout_xx to .clientdata_output_xx
Allow shinyServer() to take clientData argument
Create ShinySession$sendFile() and use from renderPlot()
Rename ShinySession to saveFile
Add plotPNG function
renderPlot: send height and width along with image
Add renderImage function
Merge pull request #109 from wch/retina

Add clientdata channel for sending data

This branch includes support for Retina-resolution displays.