Refactor udiskie.tray

Introduce a class based design. The basic algorithms remain unchanged.
Add AutoTray class

TrayIcon variant that automatically hides when there is no action
available.
Merge branch 'master' into autohide-tray

Conflicts:
	doc/udiskie.8.txt
Update documentation for 'tray' config item
Make label show up correctly in tray again

Line that was dropped during refactoring.
Merge pull request #10 from coldfix/autohide-tray

Hide tray icon when no actions available