add options for colorized log messages and uppercase log levels

Hi,
Thanks for Lumberjack. I think your design approach is very cool!

I've added some support for colors and uppercase log levels.

One other changes in here: I removed the "repr" function on the string message, since it was causing some display issues when printing Exprs. This change also removes the double quotes around the strings, which aren't probably needed.

Let me know what you think.

- Chris