Add configuration tests

[rorheden]

We need to add extensive configuration tests in order to avoid regression related to configuration.

Things that we need to test for:

• Loading of the default config.yml configuration.
• Ability to load configuration from config.yml or config.json files.
• Ability to load configuration from environment.
• Ability to load configuration provided to init(...).
• Loading order. I.e. that configuration provided to init(...) overrides configuration loaded from environment, that overrides configuration loaded from files.
• Ability to set configuration that is resolved at runtime, such as website: true.
• Invalid configuration:
  • Missing API key or ID.
  • Missing application HREF.