Add support for enhanced error messages.

[dfish3r]

Update AllowedCharacterRule and IllegalCharacterRule to support per-character error messaging.

Fixes #109 .

[dfish3r]

Note that changes to PropertiesMessageResolver are potentially breaking to anyone who has implemented custom error code keys that include dot (.)

[serac]

I think folks tend to paint in the existing lines when extending. Since the default error code keys don't contain dots, I would think consumers would naturally avoid them. That's to say I think the impact will be negligible to zero.

[serac]

I think this is a great feature with a clear and straightforward configuration. I had a couple of minor questions that I'd like to get answered before accepting.