challenge 01-01-02-age-in-days (finished & enhanced)

! modify #age_in_days to compute the number of days since birthdate
! modify interface to display the result of age_in_days computation
! modify #age_in_days to check if provided with valid dates elements
  (must be valid gregorian calendar dateset and cannot be in future)
  Raises an error if not, prints it and exits the interface before a
  success message displays without the actual computation.

Rubocop : found 1 offense
 * RedundantBeginBlock -> seems however I can not move 'begin' on the
   line below as i need to handle exceptions thrown by invalid dates
   in 'Date.new', and fail/raise if 'birth > today'.
Rake : passed 3/3 tests