Add a grid overlay for visual debugging

Adds a grid on top of the activity, similar to the one in the HIG:
http://wiki.sugarlabs.org/go/Human_Interface_Guidelines/The_Sugar_Interface/Layout_Guidelines

This is useful while doing the activity layout or while developing
widgets.

Use it like:

    var grid = require("sugar-web/graphics/grid");
    grid.addGrid(11); // 11px grid