Add support for gamma correction

[webD97]

Summary

Images should be able to be gamma-corrected.

DoD

There is a gammaCorrect() method in the Image class with the following signature:

Image Image::gammaCorrect(float, float);

There is a test method in test/Image.php, that covers the method(s) above and does not fail when executed.

void ImageTest::testCorrectGamma();