TargetScrollManager = function(notebook, options) {
        /**
         * Public constructor.
         */
        ScrollManager.apply(this, [notebook, options]);
    }
Gets the index of the first visible cell in the document.

First, attempt to be smart by guessing the index of the cell we are
scrolled to.  Then, walk from there up or down until the right cell 
is found.  To guess the index, get the top of the last cell, and
divide that by the number of cells to get an average cell height.  
Then divide the scroll height by the average cell height.