Made ezcImageConversionHandlerTest abstract
Fixed an error in imagemagick watermark function

The 3rd parameter passed to composite doesn't make sense as of today, and leads to
a failure about no color delegate being installed

This failure is made apparent by the unit tests, and are fixed by this change
Merge branch 'fix-imagemagick-watermark' of https://github.com/bdunogier/ImageConversion into bdunogier-fix-imagemagick-watermark