Give access to the dateGenerator via $.plot.dateGenerator

This allows me to format dates in a hover or click event easily and have them in the same timezone as the ticks.

var time = $.plot.dateGenerator( item.datapoint[0], options.xaxis );
time = $.plot.formatDate(time, '%Y-%m-%d %H:%M:%S%P');

I have not followed the project much, so don't know if this is acceptable or wanted, but I added this patch locally so I could use it.