AddRel() should be declared where struct Link is (package namespace 'models')

[Daniel Weck]

https://github.com/readium/r2-streamer-go/blob/master/models/metadata.go#L126

func (link *Link) AddRel(rel string) {
	...
}

...should be moved to:
https://github.com/readium/r2-streamer-go/blob/master/models/publication.go#L38

type Link struct {
        ...
}

[Bastien Quelen]

Is it was forget when we split the file i think.