Added a posterize filter for simplecv-js

![ed](https://f.cloud.github.com/assets/3799830/454070/1d001200-b32f-11e2-989f-4ddcd3eccc0b.jpg)
![edposter](https://f.cloud.github.com/assets/3799830/454071/2da8905a-b32f-11e2-9e47-8f2284b476ff.png)
![sb](https://f.cloud.github.com/assets/3799830/454075/5b12df64-b32f-11e2-9dfe-b166756bb36e.jpg)
![sbposter](https://f.cloud.github.com/assets/3799830/454078/6586eea4-b32f-11e2-95df-0188b7f43e72.png)
![sun](https://f.cloud.github.com/assets/3799830/454081/80e211ec-b32f-11e2-8726-906ef96fac89.jpg)
![sunposter](https://f.cloud.github.com/assets/3799830/454089/c0f8ced8-b32f-11e2-923f-0864afb1b570.png)
![hand](https://f.cloud.github.com/assets/3799830/454093/dd132f32-b32f-11e2-9d79-f08c6490c7de.jpg)
![handposter](https://f.cloud.github.com/assets/3799830/454097/eb4c96ec-b32f-11e2-962a-78b8383908e7.png)