Add download option.

Unlike upload option, download pulls down whole tree. For this reason
--to is expected to be directory path.
Allow --list and --download to pick specific subdirectories.

Note that in a way it introduces inconsistency with help message.
This is because arguments to --list and --download are optional
and has to be passed with '=', e.g. --list=/Documents.
Merge pull request #67 from dot-asm/master

Add download option.