Added timestamps in Chatlog

It was a little odd that timestamps checkbox in Preferences affected
only regular chat messages, but not the chatlog, so I decided to add
timestamps into the chatlog.

I hate to use `const_cast` to add a timestamp to a received message, so
if someone knows a better solution, don't hesitate to mention it.

![chat_log_timestamps](https://f.cloud.github.com/assets/1855294/458366/28683262-b3d4-11e2-804a-dc3a0a683eb7.png)