recovery: improved Time class

Added ability to convert human readable time differences to Time
instances.
Also added pretty-printing of time in __str__ method via strftime()
recovery: added ability to specify time as time difference

--time now can be specified as time difference relevant to now() via
human readable string, e.g: `1w 12h`

This is useful for crontab scripts which for example can merge all data for
last week only.