HG-250: Add timestamps to comments

This PR adds new module under `Mercury.Utils.DateTime` with currently one method `timeAgo`. Also there is a new handlebars helper utilizing the new method called `timeAgo`, which gives textual representation of time difference between past and current date.

The point of the helper is to be used on showing how far from the current date a comment was made.
New translation strings are also added for the time intervals. 
That may be tricky. i18next supports multiple setups for plurals (http://i18next.com/pages/doc_features.html) `multiple plural forms`, but I wonder how this will translate in Crowd In.