Added two distance-based types of errorPixelTransform

This pull request is related to #4 .
It adds two types of errorPixelTransforms, that show distance between color of a pixel on two images.
New options are also added to demo page.

The names (flatWithDistanceBasedIntensity and movementWithDistanceBasedIntensity) are probably not the most catchy names in the world, so I'm open to suggestions on how to change them.