provide sms notifications via messagebird

[florianholzapfel]

Description:
This PR provides a notification plugin for MessageBird (https://www.messagebird.com), a SMS gateway.

Related issue (if applicable): #

Example entry for configuration.yaml (if applicable):

Checklist:

• Local tests with tox run successfully.
• TravisCI does not fail. Your PR cannot be merged unless CI is green!
• Fork is up to date and was rebased on the dev branch before creating the PR.
• Commits have been squashed.
• If code communicates with devices:
  • New dependencies have been added to the REQUIREMENTS variable (example).
  • New dependencies are only imported inside functions that use them (example).
  • New dependencies have been added to requirements_all.txt by running script/gen_requirements_all.py.
  • New files were added to .coveragerc.
• If the code does not interact with devices:
  • Tests have been added to verify that the new code works.

[fabaff]

You could use validate_config here to check the presence of required entries.

[balloob]

Perect. 🐬