breaks on pages with no CSS rules

[glasserc]

Line 58 reads:

  forEach.call(sheet.cssRules, findDeclarations);

On <style> tags which lead to empty stylesheets, sheet.cssRules is null, which causes an error on my iOS7 device. If a stylesheet (for whatever reason) is empty, we should probably just skip the sheet:

  if (!sheet.cssRules) return;

[rodneyrehm]

thanks, fixed in 0.2.2