Make Trello / Toggl / Rethink Optional

[grailian]

The purpose of this is to allow people to launch Goldenboy with the Heroku Deploy button, without requiring them to setup API keys for Trello and Toggle, or install a RethinkDB server.

Acceptance Criteria:
If the relevant ENV vars are set:

• That module should be included
• The related commands should be triggerable
• The related commands should be displayed via the "help" command
• The related commands should be displayed via the web interface

Otherwise:

• Those modules should be ignored, and not cause any errors
• The related commands should be ignored
• The related commands should not be displayed via the "help" command
• The related commands should not be displayed via the web interface