fixed ggtree code

[GuangchuangYu]

time_scale parameter was removed since there are many time format and their positions in tip labels are diverse. We change the parameter to mrsd stand for most recent sampling date. The date will be parsed as numerical number by default. We can set as.Date=TRUE and the date will be parsed as Date.

geom_point(subset=.(isTip),aes(shape=host,color=host),size=4)+ # tip symbols 

was change to:

geom_tippoint(aes(shape=host, color=host), size=4) + # tip symbols

We provide geom_tippoint that already do the subsetting. I think it's not a good idea to expose subset=.(isTip) to new user since this feature will be removed in next release of ggplot2 (see http://ygc.name/2015/09/24/subsetting-data-in-ggtree/).