Add option to overlay Pixel Grid

Many games appear very blurry because the NDS has a much lower resolution than the iPhone everything get scaled up. Additionally the newer iPhones with Retina displays double the size once again making games look even more blurry. 
This completely is a cosmetic problem but it can be fixed/reduced by adding a grid that only shows every second pixel of the screen. The only downside of this is that brightness is reduced.

This idea was pioneered by Louie Mantia; read more about it here: http://mantia.me/blog/retro-display/. Here is the most important part:

> Answer: the “screen door” effect. When you look through a screen door, you see through the holes, and your brain fills in the missing information.

> On the older iPhone display, not only were fewer pixels shown, there was also a visible gap between pixels. By drawing semi-translucent black lines every other pixel (both vertically and horizontally), I was able to simulate the lower resolution of the old displays.

Here are some screenshots for comparison (Make sure you view them on a Retina device):
![image-1](https://f.cloud.github.com/assets/358992/858784/ada1cc82-f56e-11e2-9149-26a9b5a5c2d6.png)
![image-2](https://f.cloud.github.com/assets/358992/858783/acdf9248-f56e-11e2-8002-81e02c47ca2e.png)

More screenshots:
![image-3](https://f.cloud.github.com/assets/358992/858793/c815b20e-f56e-11e2-8105-43f95b39e24b.png)
![image-5](https://f.cloud.github.com/assets/358992/858792/c814a12a-f56e-11e2-8e17-2719540f1c7c.png)
(Landscape + Pixel Grid show the game at original resolution)