FAQ / question about ggplot

Following issue #1600: added a few lines to FAQ about ggplot. 

To me it was quite confusing that in early releases there was an implementation of ggplot, but it was not documented and later disappeared. Since that old [video](http://www.pyvideo.org/video/1224/bokeh-an-extensible-implementation-of-the-gramma) is still online, I think it would be good to clearly state that Bokeh does not and will not contain implementation of ggplot.