Use public gateway for small files

[hackergrrl]

To offset the cost of starting up a new ephemeral node (high), first ping a public gateway (e.g. ipfs.io) to get the size of the requested object. If it's over some size (recursively), start up the node. Otherwise perform a GET over the gateway (and validate the hash).

We'll soon-ish have geo-aware DNS which will make this even more useful

[hackergrrl]

@lgierth: is there a tracking issue for this somewhere?

@noffle just made one: ipfs/infra#153

I haven't looked into it at all yet, feel free if you want to :)