Update status bar item to use template image

Fixes #229.
![dark](https://cloud.githubusercontent.com/assets/2727279/3621820/c4f7911c-0e1f-11e4-8437-5c3c4af7ce00.png)
![light](https://cloud.githubusercontent.com/assets/2727279/3621821/c50bec3e-0e1f-11e4-914d-e12f2897509c.png)

You'll most likely want to recut the image itself and tweak the drop shadow a bit to make it look great in both design aesthetics.
Could you just add some spacing here?

    NSImage *statusImage = [[NSImage alloc] initWithContentsOfFile: [NSBundle.mainBundle pathForImageResource:SpectacleStatusItemIcon]];

    [statusImage setTemplate:YES];

    _statusItem.image = statusImage;