Update js/grid.base.js

Scroll automatically the grid when virtual scroll is active and a page number is set in grid's options.
See http://www.trirand.com/blog/?page_id=393/help/virtual-scroll-ajax-xml-data-go-to-page-programmatically/
Merge pull request #424 from laurentgoncalves/patch-2

Update js/grid.base.js to correct bug when virtual scroll is active and initial page number is not = 1