copy: Implement recursive copying if src is a directory.

If src param to copy is a directory, all files under it are collected
and pushed one by one to target. Source dir path handled in a way
simalar to rsync: if it ends with slash, only inside contents of
directory are copied to destination, otherwise the dir itself is
copied (with all contents of course). Original idea and implementation
by https://github.com/ansible/ansible/pull/1809 . Rewritten to address
review comments and simplify/correct logic.