Fix #62: make images resize when the sidebar collapses/expands

[bborgesr]

Test app:

library(shinydashboard)
library(shiny)

shinyApp(
  ui = dashboardPage(
    dashboardHeader(),
    dashboardSidebar(),
    dashboardBody(plotOutput("plot1"))
  ),
  server = function(input, output, session) {
    output$plot1 <- renderPlot({ plot(cars) })
  }
)