Get authorization resource with Launchpad client.

This pull request adds a Launchpad client which hits the endpoint `https://launchpad.37signals.com/authorization.json`.

You would use the Launchpad client when you need to know which accounts a user has access to. It also provides the expiration time of the token in use.

Here's an example of the JSON returned:
```json
{
  "expires_at": "2012-03-22T16:56:48-05:00",
  "identity": {
    "id": 9999999,
    "name": "Jason Fried",
    "email_address": "jason@37signals.com",
  },
  "accounts": [
    {
      "product": "bcx",
      "id": 88888888,
      "name": "Wayne Enterprises, Ltd.",
      "href": "https://basecamp.com/88888888/api/v1",
    },
    {
      "product": "bcx",
      "id": 77777777,
      "name": "Veidt, Inc",
      "href": "https://basecamp.com/77777777/api/v1",
    },
    {
      "product": "campfire",
      "id": 44444444,
      "name": "Acme Shipping Co.",
      "href": "https://acme4444444.campfirenow.com"
    }
  ]
}
```