Implement click and hover events on static plots

plotOutput now takes clickId and hoverId params that tell Shiny
where to send click and hover events for that plot. The server.R
file can listen on input$<clickId> and/or input$<hoverId>. In
both cases, the resulting value will have numeric x and y elements
that indicate the mouse position in user coordinates. In the case
of hover events, it's also possible to have a NULL value which
means the mouse is not currently hovering over the plot.
Retina display compatibility
Fix Retina compatibility (revert b113119)
Make hover delay configurable
Use crosshair cursor when plot supports hover/click
Merge pull request #183 from jcheng5/plot-mouse-events

Click and hover on static plots. Also, fix retina compatibility and make hover delay configurable