Add panTo method - paper.panTo()
Squashed commit of the following:

commit 95b03b4fae2fff814875391ff835c27e1d53c0f9
Author: Huei Tan <huei90@gmail.com>
Date:   Thu Aug 14 10:02:44 2014 +0800

    Update panTo method

commit ca9f7578679f5ca810b07f4c66d46a7c63bbcba1
Author: Huei Tan <huei90@gmail.com>
Date:   Wed Aug 13 00:08:29 2014 +0800

    start panto