Add ConfigurationDefaults and abstract sender checks.

I'll get to standardizing PlayerHandler.print() later, but I've
switched over a few messages already. Reader.readStaticMessage() is
still there in case we need to print an object with parameters (unless
we set .print up with that) or if we need to log to console.

Also add ConfigurationDefaults to set up a safety net in case we update
the config in later releases.