add onOrder property

You can select pictures sequentially.
![preview](https://cloud.githubusercontent.com/assets/530060/3548830/6db76014-08b9-11e4-836c-dabf86a20f50.png)


```
ELCImagePickerController *elcPicker = [[ELCImagePickerController alloc] initImagePicker];   
elcPicker.maximumImagesCount = 4;   
elcPicker.returnsOriginalImage = NO; //Only return the fullScreenImage, not the fullResolutionImage   
elcPicker.imagePickerDelegate = self;   
elcPicker.onOrder = YES;   //set Yes.
```