Header and footer in plot

Hi Hadley,

I just implemented the header and footer feature for the plots. There are now new themes for plot.header and plot.footer. They default to grey50 with a relative size of 0.8 and are left aligned

I changed ggplot_gtable so that a header grob is added at the top and a footer grop is added at the bottom of the tables. The text in them come from the labels "header" and "footer", 

would be great if you could merge it into the package.

Thanks

Holger