Emit warning when an insecure URL is used to download files

[flavorjones]

I would like mini_portile to emit a warning message when it downloads files over 'http'. Something like this would be great:

WARNING: insecure source being used: #{url}

[larskanis]

This is what the digest value is for. So I think it would be better so print:

WARNING: insecure source being used: #{url} - Please define a MD5/SHA1/SHA256 digest value.

[flavorjones]

Ah, I agree. We should only emit this type of warning if a checksum is not provided.