Add logging

[waynebeaton]

From an end user's perspective, the license tool is effectively a black box. We need some logging.

Minimally, we should log every interaction with a service. Configurable, finer grained logging should permit a more detailed record of the decisions made by the tool.

The implementation should take into consideration that the CLI is just one example of how the tool will be used and that the core functionality may be leveraged in other ways; e.g. a Maven Plugin (#7). Consider leveraging dependency injection (#8) in this implementation.

I believe that SLF4J is the current gold standard for logging. Convince me I'm wrong.

[waynebeaton]

This was addressed with #29.