Code Cleanup from Mandrill migration

• Removed a lot of duplicate MailchimpTransactional mailchimp_transactional from namespaces, submodules
• Renamed long machine names to mc_tx_**** such as mc_tx_template_map
• Got tests in the base module running but not in the submodules yet (will make issue or fix before release)
• README updates to fit new TXT pattern
• Tried to clean out unused code, but probably still a bunch (future deletes going into deletes branch for now)
• Install hook brings over config from Mandrill base module, including config entities template_map and activity_entity
• Tests cleaned up with more accurate descriptions, and a central TestBase for common needs.

@gcb Ready for review!

I opened this other merge request thinking it might be easier to see the file changes (because it is merging into a non-functional branch with the file names already changed).