Allow properly scrolling when grid has autoheight and OnDemand collection

[akaJes]

issue with full storage loading
its can be tested on https://github.com/SitePen/dgrid/blob/master/test/Rest.html
with adding to creation object class:'dgrid-autoheight'
or set after creation with code window.grid.set('class','dgrid-autoheight');

with this patch grid loaded properly by pages
i used function cumulativeOffset for calculating relative position to window(or parentNode )
maybe dojo has function intended for it?

[akaJes]

Hi, what do @kfranqueiro think about this patch? maybe You have another simples way to decide issue with full data loading?

[akaJes]

@kfranqueiro this path can be used for solving the last note on the page https://github.com/SitePen/dgrid/blob/master/doc/usage/Styling-dgrid.md