Adding scroll and zoom listeners

[aquintiliano]

This commit adds 2 more listeners that I need in a project:

ScrollListener (differs from PageScrollListener, since I need X and Y position)
ZoomListener

If you are not ok with the code, please, let me know, otherwise, please, merge it

Thanks

[vimalcvs]

  pdfView = findViewById(R.id.pdfView);
    pdfView.setMinZoom(1f);
    pdfView.setMidZoom(2.5f);
    pdfView.setMaxZoom(10.0f);