Added null check for canvas in UpdateVisuals method
Added a DependencyProperty FlipColorScheme, which controls the direction of the color rectangle.  This allows the color axis colors to be aligned with  texture coordinates without having to create a seperate color brush.  Addresses issue #69
Merge pull request #77 from dgwaldo/master

ColorAxis - Issues #69 & #70 - Canvas is null fix, and Allow flipping of color axis for canvas controls