Add the 'age' command for displaying contacts' ages

Incidentally, Ruby appears to lack an equivalent to PHP's DateInterval class.
There's time_diff, but that's been broken for over a year
(https://github.com/abhidsm/time_diff/issues/2). In the end I gave up and went
with a code snippet from Stack Overflow, which is the sort of programming
practice that I despise. Ruby's "optimisation for programmer happiness" isn't
look quite so optimal right now.
Version bump for "age" command
Merge branch 'age'