change dates in lifelines people to be more R-like

also corrected George Washington data of death to 1799-12-14
in R did conversion by
data[,2:5] =
lapply(lapply(data[,2:5]/1000,FUN=as.POSIXct,origin="1970-01-01"),FUN=format,"%Y-%m-%d")
removed rownames from lifelines people csv
update lifelines example and allow lifelines.json to accept string date

since we are using R, I thought R friendly dates would be more easily
understood.  I converted the milliseconds since 1970-01-01 Javascript
format to string dates in the format %Y-%m-%d.  To accommodate this, I
changed the lifelines.json spec to parse these dates.
Merge pull request #20 from timelyportfolio/master

change dates to string dates for lifelines example