Use scala logging instead of "Logging" trait

After we switch to Scala 2.11, we should look at https://github.com/typesafehub/scala-logging. The Logging trait is a little annoying in that its methods sometimes clash, and require ugly workaround (like having a separate object just to provide logging).

Also, the logging library is faster (because it uses macros and thus avoids instantiating a small closure for each logging call).