To be used in signing cookies. See PR for details.

This allows it to be backwards compatible with the existing gem, as
discussed in #5

## Usage:

```ruby
# In a controller, before serving the images
before_action :set_cookie

def set_cookie
  signed_params = AWS::CF::Signer.signed_params(
                    nil,
                    resource: 'http://images.example.com/*',
                    expires: 1.day.from_now
                  )

  signed_params.each do |name, value|
    cookies["CloudFront-#{name}"] = { value: value,
                                      domain: :all,
                                      httponly: true }
  end
end
```