Escape single quotes to be HTML5 compliant.

HTML tag attributes in HTML5 can use single quotes (') as delimiters.

Patch by Tom Lynn.

examples/apostrophe.html

@@ -0,0 +1 @@
+{{apos}}{{control}}

examples/apostrophe.js

@@ -0,0 +1 @@
+var apostrophe = {'apos': "'", 'control':'X'};

examples/apostrophe.txt

@@ -0,0 +1 @@
+'X

mustache.js

@@ -244,11 +244,12 @@ var Mustache = function() {
     */
     escape: function(s) {
       s = String(s === null ? "" : s);
-      return s.replace(/&(?!\w+;)|["<>\\]/g, function(s) {
+      return s.replace(/&(?!\w+;)|["''<>\\]/g, function(s) {
         switch(s) {
         case "&": return "&amp;";
         case "\\": return "\\\\";
         case '"': return '&quot;';
+        case "'": return '&#39;';
         case "<": return "&lt;";
         case ">": return "&gt;";
         default: return s;