Idiomatic "DefaultShell"

[jbenet]

In most tools, i want to:

• default to use a locally available ipfs node
• default to use the global ipfs-gateway

in these cases, it would be nice to have something like:

import ipfs "github.com/ipfs/go-ipfs-api"

ipfsNode, err := ipfs.DefaultNode()

that does this for me.