Add Foursquare::Location#cc

Foursquare location responses now contain country code information indexed at "cc". This commit adds a method to access this country code.

See sample response below:
```js
response: {
  venue: {
    id: "4a53aaf8f964a52098b21fe3",
    name: "Viget Labs",
    contact: {
      phone: "7038910670",
      formattedPhone: "(703) 891-0670"
    },
    location: {
      address: "400 S Maple Ave., #200",
      crossStreet: "at Tinner Hill Street",
      lat: 38.88072205833613,
      lng: -77.1767261216901,
      postalCode: "22042",
      city: "Falls Church",
      state: "VA",
      country: "United States",
      cc: "US"
    }
  }
}
```