Set the color which marks the differences

I also modified the `ImageDiffer` class. I think the `diffSizeTrigger` field duplicates similar field in the `ImageDiff` class and should not affect `ImageDiffer`'s behaviour at all. `ImageDiffer` should provide the `ImageDiff` object. Once we get the `ImageDiff` object we are free to decide which `diffSizeTrigger` value is appropriate.