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

@@ -1,17 +1,18 @@
 {
-  "curly":    true,
-  "eqeqeq":   true,
-  "evil":     true,
-  "forin":    true,
-  "freeze":   true,
-  "immed":    true,
-  "latedef":  "nofunc",
-  "laxbreak": true,
-  "loopfunc": true,
-  "newcap":   false,
-  "noarg":    true,
-  "noempty":  true,
-  "nonew":    true,
-  "trailing": true,
-  "-W082":    false
+  "curly":     true,
+  "eqeqeq":    true,
+  "evil":      true,
+  "forin":     true,
+  "freeze":    true,
+  "immed":     true,
+  "latedef":   "nofunc",
+  "laxbreak":  true,
+  "loopfunc":  true,
+  "newcap":    false,
+  "noarg":     true,
+  "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.
    *