Added setHasOne functionality (#12)

lib/manager.js

@@ -324,6 +324,30 @@ Manager.prototype.setBelongsToMany = function(model, key, models, relation, opti
   });
 };
 
+Manager.prototype.setHasOne = function(model, key, value, relation, options) {
+  var Target    = relation.relatedData.target;
+  var existing  = Target.forge(model.related(key).attributes);
+  var target    = Target.forge(value);
+  var fk = relation.relatedData.foreignKey;
+
+  if (value !== null) {
+    if (existing.get(fk)) {
+      existing.set(fk, null);
+    }
+    else{
+      target.set(fk, model.id);
+      return target.save();
+    }
+
+    return existing.save()
+      .then(function(){
+        target.set(fk, model.id);
+        return target.save();
+
+      });
+  }
+};
+
 Manager.prototype.setHasMany = function(model, key, models, relation, options) {
   var existing = model.related(key);