GraphicsMagick support with fallback to ghost utilities 

This PR adds optional support for GraphicsMagick for visual image testing, using a 'fallback' approach that tests if GraphicsMagick is available, and uses it if so.   

I've also added generation of a visdiff' file on failed tests that highlights exactly what the visual change is.  This is only supported via GraphicsMagick, but it could be implemented in GhostDiff and easily plugged in.

Finally, one extremely valuable contribution *not* on this pull request is the ability to 'warn' on visual differences, instead of failures.  This is extremely valuable for developers who expect some changes, but want to discover unexpected changes.