add onOrder property

You can select pictures sequentially.
![preview](https://cloud.githubusercontent.com/assets/530060/3548860/370d37ea-08ba-11e4-961e-032ef8c2c414.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.
```