change dates to string dates for lifelines example

For the lifelines vega spec example, I changed the dates in lifelines_events.csv to more readable string dates.  This required a slight change to the lifelines.json spec to parse these dates.

Completely unimportant, but I also discovered that George Washington's date of death was 1799-12-24 in the original vega data rather than the correct 1799-12-14.