Re-introduce SL4J logging interfaces.

As mentioned in !31 (comment 3249526), without these I get the following warnings:

SLF4J: Failed to load class "org.slf4j.impl.StaticLoggerBinder".
SLF4J: Defaulting to no-operation (NOP) logger implementation
SLF4J: See http://www.slf4j.org/codes.html#StaticLoggerBinder for further details.

The tests don't show this warning, so I'd have to do some work to figure out how to reproduce the issue in CI.