Webhook Handlers

Adds an API to simplify the handling of webhook requests.

Example usage:

```ruby
notification = Recurly::Webhook.parse(xml_body)

case notification
when Recurly::Webhook::NewAccountNoficiation

  # A new account was created
  puts "New account #{notification.account.account_code}"

when Recurly::Webhook::NewSubscriptionNotification

  # A new subscription was added
  puts "New subscription from #{notification.account.account_code}"
  
  # Notifications are not necessarily current. ! relation methods will
  # fetch the latest data
  puts "Current state #{notification.subscription!.state}"

when Recurly::Webhook::SubscriptionNotification

  # A subscription-related notification was sent
  puts "Generic subscription notification by #{notification.account!.account_code}"

end
```