Implement fetch command

It would be nice to provide a way to download files on the remote machine (in contrast to the currently implemented download command, that is downloading a file from the remote machine to the local machine).

I would call this command fetch for now.

Just dumping all the ideas here:

• check if File::Fetch is available in the remote machine, and if yes, use that
• otherwise check for curl and wget
• ideally only download the file if it's not already downloaded
• ideally it can resume an aborted download
• if there is md5/sha checksum file available at the source location, try to download them and check the downloaded content against them
• if keys are provided and gpg is available on the remote machine, try to also look for signature files, and verify them as well
• return the full absolute path of the downloaded file upon success
• throws exceptions otherwise

The main points are:

• easy downloads on the remote machine
• verify downloaded content
• do it smartly (don't download twice, resume aborted download)

I would probably hack on this soon-ish, but feel free to beat me to it if you already have something like this implemented locally.

Start by locating the existing download command and reading how remote commands and file transfers are handled. Define the scope of fetch, including remote downloader selection, resume and checksum behavior, and its success or failure result. Done should cover the requested remote download and verification cases with tests.