Improve brightness and contrast algorithm

[olivierlesnicki]

Your library is amazing. I've played around with the effects and noticed that "Brighten" and "Contrast" algorithms where different from what Photoshop layer effects output. I've decided to improve on your effects and think it might be interesting to implement them.

Check my gist https://gist.github.com/8dbdedc2f2b88f07e1c8

[alexmic]

Awesome! I'll take a look and integrate them as soon as possible!