Adding FaceAwareFill to specs

First version of the category ‘UIImageView+FaceAwareFill’
This category applies Aspect Fill content mode to an image and if faces
are detected it centers them instead of centering the image just by its
geometrical center.

More info here:
https://github.com/Julioacarrettoni/UIImageView_FaceAwareFill
Merge pull request #6948 from Julioacarrettoni/master

Adding FaceAwareFill to specs