Allow 1 remote_path to result in multiple fetched files

[oxinabox]

The default fetch_method is for the HTTP protocol,
which one where 1 path aways leads to 1 file.

But for other mechanism allowing 1 path to result in many files seems reasonable.
Which would be indicated by the fetch_method returning a Vector of local path names.

I think this makes sense for many protocols.

It would clean up the Google Drive problem (https://github.com/oxinabox/PyDrive.jl/blob/master/src/proto.ipynb)
where to download a folder you need to query the folders contents during register,
rather than during fetch
(cc @yakir12)

I think it would also make sense,
in that it would let you download a whole S3 bucket,
or recurrively fetch over FTP to get a directory.

This might actually already work.
I've not tested it.
If it does work, it is an emergant feature.