Add rel and rels convenience functions

[liddellj]

Internally at Linn, we use convenience functions for retrieving the href value of a given relation by name.

Backbone.Model.prototype.rel = function (name) {
    if (!this.has('links')) {
        return null;
    }

    var link = $.grep(this.get('links'), function (l) { return l.rel == name; });
    if (link.length > 0) {
        return link[0].href;
    }

    return null;
};

This would be a useful addition to backbone.hypermedia, in addition to a rels function which would return an array of href values based on name:

Backbone.Model.prototype.rels = function (name) {
    if (!this.has('links')) {
        return [];
    }

    return $.grep(this.get('links'), function (l) { return l.rel === name; });
};

The implementations above are pulled from two separate code bases - in adding this to backbone.hypermedia, I'm sure we can implement the rel function by making use of the rels function to avoid duplication.