Add calcs for rel freq of injury, given deer involvement

Results as of 2014-12-28: If you are in a car accident in Mt. Lebanon,
you are less likely to be hurt -- by a factor of about 10! -- if the
accident involved a deer.

> ### If there's a car accident in Mt. Lebanon, how much more (or less) likely
> ### is it to injure someone, given that a deer was involved?
> frequency <- function(bools) sum(bools) / length(bools)
> (freq_of_injury_in_all_car_accidents <- frequency(car_accidents$injuries > 0))
[1] 0.35
> (freq_of_injury_in_deer_car_accidents <- frequency(deer_accidents$Person.Injured == "X"))
[1] 0.03821656
> (rel_freq_of_injury_in_accidents_if_deer_involved <-
+      (freq_of_injury_in_deer_car_accidents /
+           freq_of_injury_in_all_car_accidents))
[1] 0.1091902