fix ie8 support

I will deploy this on the docs site for some testing before merging back to master
needed to add a getSizingRow() function for IE8 only.
fix an IE8 bug 

where scrolling past a table and scrolling back would not cause the header to show up if mixed mode or fixed positioning was used. 

fixes Issue #14
fix destroy bug
Merge pull request #15 from mkoryak/ie8fixes

fix IE8 support which I broke in 1.0.0