add xmas_countdown.rb and rspec structure
add Countdown class
it knows when is Christmas
green
it knows when is next Christmas date
green
#next_xmas is the next Christmas date
green
#today should be a date
#today is today date
green
1 day passes... test gets red
stub Date.today to be green again
#days_left should return 263
green
1 year passes... test get red
#year should return the current year
green
change hardcoded year in specs makes the test red
generalize spec date