Add db:mv function for copy and move of documents

[zadean]

@zadean
If it's not to late for this feature set, I would like to see a db:mv ,
which like the unix variant could be used for renaming or moving stored items.

I know, this would be possible with, get, delete, put (got resource) cycle,
but I think it would be nice convenance function

Originally posted by @grantmacken in #25 (comment)