Implement category for UIImage to use UIImageWriteToSavedPhotosAlbum wit...

`UIImageWriteToSavedPhotosAlbum()` requires following params.

```
void UIImageWriteToSavedPhotosAlbum (
   UIImage  *image,
   id       completionTarget,
   SEL      completionSelector,
   void     *contextInfo
);
```

So I implemented `bk_writeToSavedPhotosAlbumWithBlock` as a category of `UIImage` to pass block instead of `completionSelector`.