Mailchimp Api Versions Save

Lots of small tweaks and code improvements from pull requests. No major functionality changes. Just keeping things tidy and updated.

Also includes some internal refactoring.

This release adds a new class Webhook for receiving webhooks from the MailChimp API. It uses a pub/sub model whereby you subscribe to the webhook event with a callback function.

After developing this in a branch for a while, it's time to make the version of this for MailChimp API v3 the master branch.

I can't see any reason to carry on with the previous version when MailChimp have deprecated that version of their API, so going forward we're all in on v3. Confusingly that's v2 of this wrapper. What can you do.

Thanks so much to everyone who has contributed to this release.