Use SLF4J
Created by: NiklasRosenstein
Builds on #11 -- don't merge this before that PR.
SLF4J can be used with a number of logging frameworks as the backend, eg. [log4j], [logback] or [JUL]. Currently, it is configuerd to use JUL (so not much change there),.
All calls to Logger.getLogger() have been replaced with Main.logger instead. Eventually, we should put it into it's own Logger class instead of always importing the Main class.