rootio: add support for reading ROOT files over http/https

See: scikit-hep/uproot#50.

It would be great if rootio had also support for reading ROOT files over http.

A possible angle to tackle this would be the following:

• issue req.Header.Add("Range", "bytes=0-1023") byte ranges to get the TFile header and footer (to decipher the StreamerInfos)
• in parallel, stream in the whole file

perhaps use:

• https://github.com/jfbus/httprs
• https://github.com/DHowett/ranger

In the meantime, just download the whole file and store it locally under $TMP.