commento.js: define new thresholds for time ago descriptions
Implement new thresholds to calculate and display the age of comments. Based on the spec on issue #214 (comment 286652489):
under 120 seconds: use seconds as the unit
2-120 minutes: use minutes
120 minutes to 48 hours: use hours
48 hours to 60 days: use days
60 days to 24 months: use months
everything beyond that: years
Closes: #214 (closed)