Minor logging tweak.
Added attribute methods to Message. Moved the example configuration stuff out of overview.md, and added a brief explanation of message attributes.
Renamed MemTraceCore::ooo_width back to issue_width (ooo_width doesn't make any sense).
Allow recursive component prototypes. Make the simulator log messages about itself as "sim" rather than "simulator", since that's its reserved name in configuration.
Added more documentation. Added example configs.