Add bilinear mode for rotate()

```lua
image.rotate([dst,], src, theta, [mode]) 
```
Variable `mode` specifies type of interpolation to be used. *simple* or *bilinear*.
*simple* by default  (backward compatible)

![simple](https://cloud.githubusercontent.com/assets/287255/5552856/ab24a7c8-8c52-11e4-9b1d-29b6520414e8.png)
![bilinear](https://cloud.githubusercontent.com/assets/287255/5552857/b04bbff2-8c52-11e4-9d95-061fd9207fc6.png)