Test on current and future core version.

- Fixes #54

Update coding standards.

- Fixes #52

Use render system when sending messages.

- Fixes #49

Ignore display mode creating if you're syncing a config folder.

Keep up with Message changes.

- Fixes #40

Do not require a message to construct a notifier.

- Fixes #32
- Makes the Message object optional when constructing a notifier, and
  adds a `setMessage()` method.
- Adds an assertion to the `send()` method to ensure an actual message
  is set when that is called.

Throw exception on no message owner

MessageNotifierInterface extends plugin interfaces.

- Fixes #33

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