API key required for MapQuest Open services

MapQuest now requires an API key for their open services, as well as their licensed APIs.

http://developer.mapquest.com/web/products/open/forums/-/message_boards/view_message/459709

> To that end, we have decided to require a free key to access all of the open services. The services still remain free and open but you will need to sign up for a key and send that key in your requests to continue to use the services. A key will be required to use the Open services after July 15, 2013.

MapQuest lookup class updated to require an API key, and to accept a new `:licensed` configuration option to specify use of licensed vs. open API.

`Geocoder.configure(:mapquest => {:licensed => true})`