Cleaner logging library usage

Our SBT project is currently dependent on both log4j and logback, while using two logging backends is not ideal.

Furthermore, there has been report that in some configuration we have both slf4j-log4j12 and log4j-over-slf4j in the classpath which leads to StackOverflowError.

The current codebase is not written with a clean usage of logging library in mind, and I propose the following improvements to make our logging usage conform with the de-facto standard of slf4j.

• Figure out how the dependencies are using slf4j, log4j, and logback.
• Use slf4j-api everywhere, named with the fully qualified class names.
• Exclude slf4j backends from the library projects
• Add either log4j or logback backends to the application projects, preferably log4j.
• Add a test case checking that the classpath has the correct configuration of jars.