added gamma filter for simplecv-js

gamma = 0.25

![0 25l](https://f.cloud.github.com/assets/3799830/452459/60bccfac-b2e8-11e2-92a8-d6df7a603ccf.png)

gamma = 0.5

![0 5l](https://f.cloud.github.com/assets/3799830/452460/6ddf2aa4-b2e8-11e2-8da5-f0499ce626e0.png)

gamma = 1 (original image)

![1l](https://f.cloud.github.com/assets/3799830/452462/7eb07acc-b2e8-11e2-8eeb-3215bf081d22.png)




gamma = 0.25

![0 25n](https://f.cloud.github.com/assets/3799830/452463/8d9b2014-b2e8-11e2-8f85-73ff6b8d9a88.png)

gamma = 0.5

![0 5no](https://f.cloud.github.com/assets/3799830/452465/980ef84a-b2e8-11e2-9f3b-6ff4f3dd9105.png)

gamma = 1 (original image)

![1no](https://f.cloud.github.com/assets/3799830/452466/a5bbdab2-b2e8-11e2-8480-a56aa4bf62ae.png)

gamma = 2 

![2no](https://f.cloud.github.com/assets/3799830/452467/b1024212-b2e8-11e2-99fa-6a0a47942d37.png)