IE8 reads from 'styleSheet', not 'sheet'.

Fixes #3.

IE8's `<style>` DOM uses `styleSheet`, but modern browsers use `sheet`.

[MSDN: `styleSheet`](http://msdn.microsoft.com/en-ca/library/ie/ms535871(v=vs.85).aspx)
[MSDN: `sheet`](http://msdn.microsoft.com/en-ca/library/ie/ff974766(v=vs.85).aspx)