Create the UnlockedAchievement class.

This commit also adds the time since each achievement was completed.

Achievements should be printed in alphabetical order. AchievementTracker should
sort the UnlockedAchievements according to their ID (as this, hopefully, will
always start the same way as the achievement's name).

Consider implementing insertion sort and using binary search to improve
getUnlockedAchievement() performance.
A little comment.