Migrate to Mailchimp Transactional API for final Release

This is a lot of work, but I'd divide it into three general categories:

1. Updating the library dependency from the old Mandrill API to the new Mailchimp Transaction API
2. Code Styles updates, mostly from code sniffer, to clean up some old approaches, enforce some types to help future troubleshooting, and just generally tidy up.
3. Deleting broken or deprecated features:

• Removed “Tracked URLs” report, due to deprecation of that API functionality ( https://mailchimp.com/developer/transactional/api/urls/list-most-clicked-urls/ )
• Removed addInboundDomain - not used in module
• Removed addWebhook - not used
• Removed addInboundRoute - not used
• Removed deleteInboundDomain - not used
• Removed getInboundDomains - not used
• Removed getInboundRoutes - not used
• Removed getWebhooks - not used
• Removed getSenders - not used
• Removed getSender - not used
• Removed getSenderTimeSeries - not used
• Removed getUrls - deprecated API functionality
• Removed getURLTimeSeries - deprecated

I am sure this will be a doozy to review, let me know what questions come up.

Also, there is a fix for this issue which I found while fixing the tests. We should credit rwanth somewhere.