Add PSR-3 logging

The `TransformationEngine` should accept a PSR-3 logger implementation and route it through to the transformers. This is a standardized way of letting the consumer of the library decide how and when to accept logging information and is mandatory in larger projects to maintain control over the different parts of the system.

The library should also include a logger implementation that can be used (or will be used by default, TBD) that logs errors into HTML comments, as a sort of last resort to communicating error conditions in an HTTP stack that doesn't come with its own PSR-3 routing.

In the future, the validator and the sanitizer engines should accept such a PSR-3 logger as well.