Added the ability to initialize a custom logging procedure using Driver's new_with_logging

[mlinden4]

I changed the events.rs file to add the Logger trait and added a field to Driver to remember which logger to initialize. If you don't want the field in Driver, you could move the logging initialization to Driver::new()/new_with_logging(). I also had to change a line in engine_simple because it initializes the logging outside of the Driver and had to be changed to support the default EnvLogger.

[JoeOsborn]

Thanks! I wonder if the dynamic dispatch could be avoided with something like: struct Driver<L:Logger=EnvLogger>{ logger:L, ... }. I don't think it's bad for there to be a logger field, it's easy to imagine a logger that needs some global configuration.