Switch from logula to slf4j for logging.

To prevent downstream dependencies from depending on a particular
logging implementation, switch from logula to slf4j for logging. Upgrade
to a more recent version of the Metrics library. Make sure the
InstrumentedThreadPoolExecutorSpec testcase runs with Maven.
Use latest 1.7.5.
Merge pull request #6 from pkwarren/use_slf4j

Switch from logula to slf4j for logging.