add gamma correct option

[lefuturiste]

With a new option on the main class, we will able to tell if we want gamma correction.

When you use this library and rescale a image, the image's colors tends to not be keeped, this is caused by the image gamma correction applied before and after the proccessing.

[coveralls]

Pull Request Test Coverage Report for Build 199

• 4 of 6 (66.67%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.4%) to 81.081%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
lib/ImageResize.php	4	6	66.67%

Totals
Change from base Build 196:	-0.4%
Covered Lines:	240
Relevant Lines:	296

---

💛 - Coveralls

[adityapatadia]

It's great addition but we should provide it as function rather than just putting a class variable.
Something like following would be great.

$image = new ImageResize('image.jpg');
$image->gamma(false);

Please also update readme file accordingly and I will merge this and release new version.

[lefuturiste]

This is the new commits I done. Should I rename the class variable gamma_correct to gamma ?

[lefuturiste]

Btw, Happy new year!

[adityapatadia]

Thanks a lot for your efforts. Happy new year 🎉