Column resize performance

I wasn't able to reproduce the missing checkbox issue in #4, but I imagine it might be fixed by this PR. I was able to confirm that this will fix the missing border strokes and the resize lag from #7. Let me know if I missed a case. The important commit is bd3e75c, but I threw some other cleanup I did as I went in here.

It took me a while to find where to fix this, but... caching...

![2014-11-03 17_41_57](https://cloud.githubusercontent.com/assets/594059/4893172/995c829e-63bd-11e4-95c3-aaf56d12b422.gif)

Fixes #4 and #7.