Make generated parsers use strict mode

* Issue #324

 * JSHint complains about two possible strict violations. But are valid
   uses of `this`, so we suppress the warnings.

.jshintrc

@@ -13,5 +13,6 @@
   "noempty":   true,
   "nonew":     true,
   "trailing":  true,
+  "validthis": true,
   "-W082":     false
 }

lib/compiler/passes/generate-javascript.js

@@ -761,6 +761,8 @@ function generateJavascript(ast, options) {
 
   parts.push([
     '(function() {',
+    '  "use strict";',
+    '',
     '  /*',
     '   * Generated by PEG.js 0.8.0.',
     '   *',

lib/parser.js

@@ -1,4 +1,6 @@
 module.exports = (function() {
+  "use strict";
+
   /*
    * Generated by PEG.js 0.8.0.
    *