sortFunction breaks IE8

[fibo]

Hi,

I open issue just to contribute back to your great work, I found this problem when using Internet explorer version 8 (and lower)

Array.prototype.sort: argument is not a JavaScript object

I don't pretend a fix, I already fixed it on my local version just changing

var keys  = objectKeys(columnLabels).sort(fields[key].sortFunction),

to

var keys  = objectKeys(columnLabels).sort(),

since i'm not using that feature.

Just wanted to notify you that IE8 will complain and in my case showed a blank page. Maybe you want to add a warning in your docs about this.