Minor improvement on Preview Widget


- Auto fit image inside preview area
- Show image size label on lower left corner
![preview_auto_fit_size](https://cloud.githubusercontent.com/assets/3735467/5422619/87bc3674-82c6-11e4-92c8-99b631ec08aa.JPG)

- Show file path for non-image asset
![preview_show_path](https://cloud.githubusercontent.com/assets/3735467/5422620/87e382ec-82c6-11e4-9b07-0879f8723f37.JPG)