Optimize performance of getVirtualItems

[jordaaash]

getVirtualState is getting the height of the container every onScroll (in addition to a lot of other repeated calculations) when the values likely haven't changed. I believe this is causing elements to be reflowed by the browser, and even if not, is doing the same work over and over.

I will try to optimize the method after #14 which is required to know if things need to be recalculated.

[jordaaash]

Also possibly related: #1, #3

[developerdizzle]

Great catch; this should be able to be cached, as long as we recalculate it on componentWillReceiveProps if the container has changed.