Allow multiple metadata values

![screen shot 2014-07-16 at 12 01 10](https://cloud.githubusercontent.com/assets/74812/3597898/d26d16ba-0cd9-11e4-9f94-a4a6f46f9321.png)

![screen shot 2014-07-16 at 12 01 14](https://cloud.githubusercontent.com/assets/74812/3597899/d33d5c62-0cd9-11e4-8f04-57b56554c34b.png)