Moved setImmediate shim to wallet.

lib/wallet.js

@@ -8,6 +8,9 @@ var SigningKey = require('./signing-key.js');
 
 var utils = require('./utils.js');
 
+// This ensures we inject a setImmediate into the global space, which
+// dramatically improves the performance of the scrypt PBKDF.
+require('setimmediate');
 
 var transactionFields = [
     {name: 'nonce',    maxLength: 32, },