Commands Refactor Part 2

We split #196 in two parts. this is the second. (#262 is the first). This builds on all the changes to commands/.
```
func cat(node *core.IpfsNode, paths []string) ([]io.Reader, error) {
	readers := make([]io.Reader, 0, len(paths))
	for _, path := range paths {
		dagnode, err := node.Resolver.ResolvePath(path)
		if err != nil {
			return nil, err
		}
		read, err := uio.NewDagReader(dagnode, node.DAG)
		if err != nil {
			return nil, err
		}
		readers = append(readers, read)
	}
	return readers, nil
}
```