[RFC] [ui] add a global menubar and docker menu on macosx platform

When the app starts, OS X will setup a global application menubar on the top of the screen.
![screen shot 2014-12-19 at 15 58 44](https://cloud.githubusercontent.com/assets/2123461/5501560/24c01762-8798-11e4-86f5-6aecf839c56e.png)

 > Ubuntu should have similar functionality, but I haven't tested on it yet.  


And when the user right-clicks the icon in the docker, we can have a menu top the icon:

![screen shot 2014-12-19 at 15 58 55](https://cloud.githubusercontent.com/assets/2123461/5501579/6c6b2070-8798-11e4-861e-9d820efde7e0.png)

Why not only initialize the `global_menubar_` and `globa_menu_` variables on mac osx? It seems strange to delete them right after initializing them.