System tray initial support
Added initial support for system tray icon. This will be disabled by default (for now), to enable this feature set variable SystemTray to True in configuration file ~/.fbmessenger/config.py . For now system tray just has two options for right click to hide/show application and to quit it. Left clicking system tray will hide/show application. For now closing the main window will quit application instead of minimize it to tray (to see if it   will be better to minimize it).