feat(tasks): gallery mode for finished tasks

![gallery-landscape](https://cloud.githubusercontent.com/assets/298275/5061782/7f8a6c9e-6da6-11e4-985f-4f895668d111.png)
![gallery-portrait](https://cloud.githubusercontent.com/assets/298275/5061781/7f730ff4-6da6-11e4-80d7-e34e77cb4e7b.png)