subgrid feature

[jpuri]

A couple of things to mention:

1. Changes in core are mostly those that are done to fix scrolling to use row.height instead of gridOption.rowHeight.
2. Row headers column currently do not have column header. This is issue with row header functionality that needs to be fixed.

[c0bra]

A couple issues:

1. The sort ascending icon now looks like an unanimated "loading" circle?
2. Intermittently, in Chrome, if I expand all rows then drag the vertical scrollbar down to the middle of the grid, the expanded grids simply do not render at all. I haven't had time to dig into a possible reason, or check it in any other browsers. So far this happens maybe 50% of the time. If I scroll enough either up or down then they will start rendering again.

Beyond that, this stuff is looking really great. It's pretty snappy, sorting isn't too bad with the grids expanded, it looks good visually, etc. I'd be curious to see this with horizontal scrolling.