Change from rack-ip-whitelist to ip_whitelist. Lots of improvements:

* include Railtie to auto-load the middleware
* added YAML support so IP list can be specified in config/whitelist.yml
* added 'all allowed' if environment is left off
* added 'none allowed' if environment tag is present but no list
* error to 'none allowed' if YAML has formatting issues
* Cleaned up readme and gemspec