Use swift-log as logging system

[filipbodor]

As we now use swift-log in slido for mac project, it makes sense to transfer to the logging system also in this swift library.

Removed:
I removed old logging system which was used by this library.

Implemented:
Instead of storing OAuth2Logger instance, the library will store Logger instance from swift-log framework.
I needed to change the way log messages were written:

logger.warn()                                 // -> logger.warning()
logger.info("OAuth2", msg: "message")         // -> logger.info("message")

[DominikPalo]

@filipbodor could you please link this PR with #24 issue to make it trackable and update the status (and other details, like "assignee") of that issue? We use this approach for all other features in this library, since we don't use Jira for open-source projects.