Convert tests to phpunit-based framework

- Fixes #26

MessageType is now MessageTemplate

- Also download the Message module from the main repo.

- Since field storages are bundled here, this module must depend on the
  field module.

Some todo and code style cleanup

Add a composer.json file

Fixes fatal error during config import

- Under certain conditions, this hook can fire before the 2 custom
  display modes are available, resulting in a fatal error.

Fix travis & tests

Initial port to 8.1