Document and automate tests

[hiddewie]

It would be nice to have a description on how to test Gnomecast, manually and automatically. This might make is easier to open a pull request, because it becomes more clear what to expect from contributors.

Concretely I would suggest:

• A readme section on how to run the test suite.
• A test suite which highlights 'features' or 'requirements' that are implemented in the application.
• Some CI that runs on every pull request and master commit. For example Semaphore CI is free and is easy to get started with. Python documentation: https://docs.semaphoreci.com/programming-languages/python/
• A code style guide 'linter' that would also run for every commit, that enforces a single code style across the code base.

[keredson]

👍 CI and unit testing are definitely needed.