Add logo for td-agent-ui


![2014-12-10 16 50 11](https://cloud.githubusercontent.com/assets/499680/5372506/11b34db8-808d-11e4-90a0-8fe465f88ebd.png)

You can see exec fluentd-ui with `FLUENTD_UI_LOGO=/td-logo.png`

@kiyoto how's that?