Use document.documentMode when detecting IE versions

Closes #1149

Author: marijnh

lib/codemirror.js

@@ -8,8 +8,8 @@ window.CodeMirror = (function() {
   // bugs and behavior differences.
   var gecko = /gecko\/\d/i.test(navigator.userAgent);
   var ie = /MSIE \d/.test(navigator.userAgent);
-  var ie_lt8 = /MSIE [1-7]\b/.test(navigator.userAgent);
-  var ie_lt9 = /MSIE [1-8]\b/.test(navigator.userAgent);
+  var ie_lt8 = ie && (document.documentMode == null || document.documentMode < 8);
+  var ie_lt9 = ie && (document.documentMode == null || document.documentMode < 9);
   var webkit = /WebKit\//.test(navigator.userAgent);
   var qtwebkit = webkit && /Qt\/\d+\.\d+/.test(navigator.userAgent);
   var chrome = /Chrome\//.test(navigator.userAgent);