[Layer] Inverted coordinate system, positive Y goes down

This is obviously optional, but I think that most of designers using Origami are used to Web Developement.
In web development Positive Y goes down, while in QC goes up. 

It's very tedious to always positions elements using -Y. That's why I just added a `Math *-1` as seen on the image

![update](https://f.cloud.github.com/assets/390010/2156912/6a55ed6c-946c-11e3-82f8-bbbb874dd10c.jpg)