Make SlickGrid more accessible.

SlickGrid doesn't work for people who use screen readers right now.  When navigating the grid with the keyboard, screen readers aren't aware that anything is happening.  This change introduces a few simple changes that make slick grid much more usable for people that have to use a screen reader:

1) focus() cells as they are made active in slickgrid.  This notifies screen readers that focus has changed so that they can read the cell (and the column header).

2) add aria-grid attributes so that screenreaders are aware the divs in slick grid make up a table.  This is usually very useful for table-y things like slickgrid, although the spec doesn't allow us to specify how many rows the grid has so it is somewhat deceptive (it will read something like "table with N columns and M rows" (where M is 40 instead of 2000, for instance).  I think this is a reasonable trade-off because it still gives blind people some contextual information about what the grid is and how many columns it has.

I'll make a few more notes in the code.