Added 'pixel effect' as a Layer parameter

New optional Layer parameter:

> **pixel effect**
> Optional dictionary that defines an effect to be applied for all tiles of this layer. Pixel effect can be any of these: `blackwhite`, `greyscale`, `desaturate`, `pixelate`, `halftone`, or `blur`.

```json
{
    "cache": { ... },
    "layers": {
        "layer_name": {
            "provider": { ... },
            "pixel effect": {
                "name": "desaturate"
            }
        }
    }
}
```

Examples of resulting tiles:

**0. no pixel effect**

![sf](https://cloud.githubusercontent.com/assets/1008637/5132973/f5b1f812-70b5-11e4-9e98-f5b581852029.png)

**1. blackwhite**

![sf_blackwhite](https://cloud.githubusercontent.com/assets/1008637/5132992/21470eb8-70b6-11e4-941d-dd5c7ffcc214.png)

**2. greyscale**

![sf_greyscale](https://cloud.githubusercontent.com/assets/1008637/5132994/2bee4db8-70b6-11e4-8f2b-cc810d3e1350.png)

**3. desaturate**

![sf_desaturate](https://cloud.githubusercontent.com/assets/1008637/5132998/34e459f8-70b6-11e4-99a5-0bc704726214.png)

**4. pixelate**

![sf_pixelate](https://cloud.githubusercontent.com/assets/1008637/5133002/395e0ace-70b6-11e4-902d-b9249ba089b2.png)

**5. halftone**

![sf_halftone](https://cloud.githubusercontent.com/assets/1008637/5133004/3e205224-70b6-11e4-9f1e-120ccd83475c.png)

**6. blur**

![sf_blur](https://cloud.githubusercontent.com/assets/1008637/5133008/48d88aec-70b6-11e4-9527-8d51b5cd2cac.png)