Fix the tray icon problem in Unity

Tray icon is displayed wrong in Ubuntu 14.04, as shown in [issue 148](https://github.com/jkozera/zeal/issues/148). Then, I use application indicator instead of tray icon in Ubuntu 14.04 following the instructions. 

Note: left-click on application indicator won't show the application. This is the feature of application indicator. We can use hotkey to show the application. 

New requirements: libgtk2.0-dev libappindicator-dev