Jeckle Request

I'm not sure if we should move `connection` to a `Jeckle::Request` or something like that, I was with that in mind at first, but it seems natural to me that `Jackle::API` should have their on connection.

We are able to:

```bash
rake build install
```

```ruby
# Ruby console
require 'jeckle'

Jeckle.configure do |config|
  config.register :dribbble do |api|
    api.base_uri = 'http://api.dribbble.com'
    api.headers = { 'Content-Type' => 'application/json' }
  end
end

dribbble_api = Jeckle::Setup.registered_apis[:dribbble]
response = dribbble_api.get 'shots/1600459'

shot = JSON.parse response.body

shot['image_url']
# => "https://d13yacurqjgara.cloudfront.net/users/85699/screenshots/1600459/daryl_heckle_and_jeckle_oates-dribble.jpg"
```

URL from shot:

![Jeckle and Heckle](https://d13yacurqjgara.cloudfront.net/users/85699/screenshots/1600459/daryl_heckle_and_jeckle_oates-dribble.jpg)