v0.2.0

1. Adds events and event subscription endpoints.
2. Adds helper methods to support webhook signature verification:

   Event event = client.webhooks().unwrap(body, headers, secret);
   client.webhooks().verifySignature(body, headers, secret); // throws if not valid

3. Adds mcc to transactions.
4. Changes how headers, body properties, and query parameters can be customized (eg, with .putHeader("X-Foo", "bar")).

Full Changelog: v0.0.4...v0.2.0