Moved from Dock to system status bar and added launch on startup

I really like your app, simple but effective :+1:

But what's been bothering me was the icon always taking up space in my Dock. So I made it go to the system's status bar instead.

I also made it possible to hide the app from the status bar (to make it completely invisible), with the ability to restore the icon by relaunching the app (like many other do)

Last but not least, I added a checkbox in the menu to make it launch at startup.

[Screenshot](https://f.cloud.github.com/assets/1523191/480143/04f39938-b826-11e2-9708-394af91bead9.png) of how it looks.

I mostly made this for myself, but if you like it, feel free to accept this PR :)