Added theme support and OS X notification tray icon

A user can now choose between several themes using either the "--theme"
switch or a config value. By default, only OS X uses its own black and
white theme.

Furthermore a OS X specific tray icon was added which supports native
notification using a little helper tool named "terminal-notifier"
(https://github.com/alloy/terminal-notifier; license MIT, free for use).
The native notifications appear in the OS X notification center
featuring a nice Syncany icon.