Implement fetch command

[ferki]

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.