Add curve filter for applying RGB curves.

## Usage

    /filters:curve(<curve of all channels>, <curve of R>, <curve o f G>, <curve of B>)

### Format of each curve

    [(x1,y1),(x2,y2),...]

### Examples of use

#### Original image
![image](https://cloud.githubusercontent.com/assets/982461/4415799/4b231498-4532-11e4-9798-c39ad95ed63a.png)

#### Demo 1
    /filters:curve([(0,0),(40,59),(255,255)],[(32,59),(64,80),(92,111),(128,153),(140,169),(164,201),(192,214),(224,215),(240,214),(255,212)],[(34,41),(64,51),(92,76),(128,112),(140,124),(164,147),(192,180),(224,216),(240,236),(255,255)],[(40,46),(64,55),(92,83),(128,127),(140,144),(164,174),(192,197),(224,199),(240,197),(255,198)])/
![image](https://cloud.githubusercontent.com/assets/982461/4415716/fd91798c-4530-11e4-963a-9c2be9f29183.png)

#### Demo 2
    /filters:curve([(0,0),(255,255)],[(0,50),(16,51),(32,69),(58,85),(92,120),(128,170),(140,186),(167,225),(192,245),(225,255),(244,255),(255,254)],[(0,0),(16,2),(32,18),(64,59),(92,116),(128,182),(167,211),(192,227),(224,240),(244,247),(255,252)],[(0,48),(16,50),(62,77),(92,110),(128,144),(140,153),(167,180),(192,192),(224,217),(244,225),(255,225)])/
![image](https://cloud.githubusercontent.com/assets/982461/4415722/160c41c2-4531-11e4-93ef-5ed821ce5ea7.png)