Refactored to support other log format easily

HI, irc-log-viewer has functionality which I want. But unfortunately my irc log file format is different from sample logs.

So I started to hack it, and found it was a bit difficult to read and extend the code.

I refactored it, and add a class `LogFormatTiarra` to support my log format. Tiarra is my irc client: http://www.clovery.jp/tiarra/

I changed a lot, so you might not like my code/style. Thanks!