Integrated OSX menubar icon
- cgo-based native interface using objective-C
- support for arbitrary menu items with callbacks

TODO:
- manage memory properly
- decide how to handle "Quit" in Go-friendly and cross-platform manner

NOTE: this is a blend of the menubar code in the client app from
github.com/xilp/systray and the darwin cgo dock approach from
github.com/cratonica/trayhost
Drop port argument
Unify windows api with darwin, allowing menu items to be specified up
front and instantiated on demand
Barebones test application that adds a few simple items to a menu
Reflect functionality changes in README