Added tray icon with visibility toggling

As mentioned in #5, we cannot add minimize to tray functionality yet. However, this is a start in the right direction. In this PR:

- Add Tray icon
- Add menu items for toggling window visiblity and exiting
- Add click functionality to tray icon to toggle window visibility.

![icon-screenshot](https://cloud.githubusercontent.com/assets/902488/5546767/f89016da-8b0f-11e4-9e7a-cca21cbe6958.png)