Allow the use of custom OnTouchListener

In https://github.com/davemorrissey/subsampling-scale-image-view it is possible to assign a custom GestureDetector that works parallel to the gesture functions of the library.

Therefore it is possible to use code like this:

``` java
mImageView.setOnTouchListener(new View.OnTouchListener() {
@Override
public boolean onTouch(View v, MotionEvent event){
// detect swipe gestures or any other gesture
}
})
```

However, this is not possible with PhotoView, since it assigns its own OnTouchListener directly in the PhotoViewAttacher constructor.

I know that there are some helper methods like `mAttacher.setOnSingleFlingListener();` or `mAttacher.setOnDoubleTapListener();`, but if would be much more convenient if the standard interface could be used.

Is there any plan to solve this issue?