don't doublt encode entities. Closes #19. Patch by Leeoniya

examples/escaped.html

@@ -1 +1,2 @@
-<h1>{{title}}</h1>
+<h1>{{title}}</h1>
+But not {{entities}}.

examples/escaped.js

@@ -1,5 +1,6 @@
 var escaped = {
   title: function() {
     return "Bear > Shark";
-  }
+  },
+  entities: """
 };

examples/escaped.txt

@@ -1 +1,2 @@
 <h1>Bear &gt; Shark</h1>
+But not &quot;.

mustache.js

@@ -208,7 +208,7 @@ var Mustache = function() {
       Does away with nasty characters
     */
     escape: function(s) {
-      return ((s == null) ? "" : s).toString().replace(/[&"<>\\]/g, function(s) {
+      return ((s == null) ? "" : s).toString().replace(/&(?!\w+;)|["<>\\]/g, function(s) {
         switch(s) {
           case "&": return "&amp;";
           case "\\": return "\\\\";;