make Message log font one step bigger

Closes #3139.
I agree that the font size in the message log is too small.

Below are not very good examples, since here it's white text on black, so it is always easy to see. But it's an improvement I think.

__Before:__
![screenshot-20140826-100656](https://cloud.githubusercontent.com/assets/619390/4041580/fc9b9b80-2cf7-11e4-87b5-d3f57c788442.png)

__After:__
![screenshot-20140826-095931](https://cloud.githubusercontent.com/assets/619390/4041567/c7400a84-2cf7-11e4-9e62-d500248af18a.png)