Migrate Cobertura off log4j and to slf4j.

There are a couple reasons for this pull request:

- Cobertura, being a library, shouldn't enforce a logging mechanism on its consumers. With the migration to slf4j-api, consumers can choose their preferred logging mechanism. (And no longer need to exclude log4j in their Maven POMs.)

- Log4J has been dead for many years. Logback is its modern and actively maintained replacement. Only test scope depends on it, and only through slf4j-api. (See above.)

- Log4J configs in main have been removed, since that's a consumer config. Log4J configs in tests have been converted to Logback using the online tool.

- All tests pass after applying this PR. (Tested with OpenJDK 7 on Fedora Linux.)