Add menu bar icon on macOS #206

It'd be cool to show the number of unread notifications and add the ability to remove the app from dock. Something like this: <img width="419" alt="dock_icon_preview" src="https://cloud.githubusercontent.com/assets/9095073/26037960/2133b7b8-38f6-11e7-8635-19ad84fab141.png">

The number may be too small in my example, but I think the main idea is there.