Add support for custom label font and color, close button and auto hide.

Hi,

Great class. Such a component is sorely missing on OSX.
I added a few bits I needed : 
* custom label font and color
* an optional close button
* support for auto hiding the overlay after a given delay

I also cleaned up the code and xcode to separate the core files from the sample app.

Cheers,
Cherif

![cap_overlay](https://f.cloud.github.com/assets/69134/66916/661e0ffe-5ed5-11e2-84ef-2d33ecb259d5.png)