Idiomatic "DefaultShell"

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:

```go
import ipfs "github.com/ipfs/go-ipfs-api"

ipfsNode, err := ipfs.DefaultNode()
```

that does this for me.