List snapshots by time

Users now have the choice of listing the snapshot by time as well as
name (default behaviour).
An additional flag has been added '--sort=' which controls the sort
method applied to the list produced by aptly snapshot list.

The possible values are:
--sort=name (default): sorts the snapshot list by name (lexicographic
order)
--sort=time: sorts the snapshot list in chronological order (oldest to
newest)
List snapshots by time: added integration tests

Added a couple of integration tests for the new list snapshot by
creation time feature.