Resolve Signature drift between NewPush and NewPull

[b5]

// Pull now accepts a meta param
NewPull(cidStr string, lng ipld.NodeGetter, bapi coreiface.BlockAPI, rem DagSyncable, meta map[string]string) (pull *Pull, err error)

// Push doesn't...
func NewPush(lng ipld.NodeGetter, info *dag.Info, remote DagSyncable, pinOnComplete bool) (*Push, error)

I think we should pull the meta param off of pull, and use the form that push uses: a SetMeta method on the push itself.