Update c3.js and c3.min.js

c3.js

@@ -273,6 +273,10 @@
     var nodeCss;
     var styleCache = {};
 
+    c3.clearStyleCache = function(){
+        styleCache = {};
+    }
+
     // Seek a specific value in the stylesheets of a document
     function getStyleValue($$, selector, property) {
         function cache(value){
@@ -318,6 +322,7 @@
         }
         for (var styleId = 0; styleId < document.styleSheets.length; styleId++) {
             sheet = document.styleSheets[styleId];
+    	if(!sheet.cssRules) return undefined;
             for (var ruleId = 0; ruleId < sheet.cssRules.length; ruleId++) {
                 rule = sheet.cssRules[ruleId];
                 if(rule.type !== CSSRule.STYLE_RULE) continue;