set the margin to 0 before plot.new()

otherwise users will be unable to draw small plots because of the common error "figure margin too large", e.g. renderPlot(..., width=200, height=100)
explain why par(mar = rep(0, 4)) is necessary before plot.new(), per @wch's suggestion
Merge pull request #508 from yihui/bugfix/large-margin

set the margin to 0 before plot.new()