PROV-743 : Watermark support with CoreImageTool

- CIStretchCrop to resize the watermark
- CIColorMatrix to modify watermark opacity
- CISoftLightBlendMode to blend images

Signed-off-by: gautiermichelin <gm@ideesculture.com>
Merge pull request #61 from ideesculture/coreimage_watermark

PROV-743 : Watermark support with CoreImageTool