Add http source

[tonistiigi]

HTTP source would give access to the archives pulled from the web. It can use headers for detecting when the contents have not changed.

[AkihiroSuda]

It can use headers for detecting when the contents have not changed.

Should we only support ETag?
Should we also allow comparing Last-Modified and Content-Length, though it is not reliable?

[tonistiigi]

I think it should mostly be based on ETags. I don't have enough data to evaluate if there are lot of cases where Last-Modified would work and ETag would not. I guess we could implement Etag first and the others later if needed.

[AkihiroSuda]

link: PR (by tonis): #168