Implemented update checking with yeoman/update-notifier

Fixes #24 - Implemented update checking with yeoman/update-notifier

Unfortunately, yeoman is *too* magical.  It seems to walk through your directories calling any function that isn't private and doesn't allow any global hooks for you to put common code across generators.  Sooo... I had to add the update check to every generator.

To test this locally:
1. ```npm link``` to swap out your global version with this version
2. Change package.json's version to 0.0.0 (less than the current version 0.0.1)
3. Run some generator commands like ```yo kraken:controller test```.  To keep the generator speedy, updates are only checked once a day.  To change this setting for local testing, modify lib/update.js

Let me know if you'd like some mocha tests.  It'd be a bit tricky.  I'd probably have to spawn a child_process, then capture stdout and look for the update notification.