added user-agent-rewriter, updated README

user-agent-rewriter/background.js

@@ -0,0 +1,30 @@
+"use strict";
+
+var targetPage = "http://useragentstring.com/*";
+
+var uaStrings = {
+  "Firefox 41": "Mozilla/5.0 (Macintosh; Intel Mac OS X 10.10; rv:41.0) Gecko/20100101 Firefox/41.0",
+  "Chrome 41": "Mozilla/5.0 (Windows NT 6.1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/41.0.2228.0 Safari/537.36",
+  "IE 11": "Mozilla/5.0 (compatible, MSIE 11, Windows NT 6.3; Trident/7.0;  rv:11.0) like Gecko"
+}
+
+var ua = uaStrings["Firefox 41"];
+
+chrome.webRequest.onBeforeSendHeaders.addListener(rewriteUserAgentHeader,
+                                          {urls: [targetPage]},
+                                          ["blocking", "requestHeaders"]);
+
+function rewriteUserAgentHeader(e) {
+  for (var header of e.requestHeaders) {
+    if (header.name == "User-Agent") {
+      header.value = ua;
+    }
+  }
+  return {requestHeaders: e.requestHeaders};
+}
+
+chrome.runtime.onMessage.addListener(setUaString);
+
+function setUaString(message) {
+  ua = uaStrings[message.uaString];
+}