Move adjustment for (left, top) if :center is set to a common routine.
Use new routine from arc, oval and rect
Added a 'center' option to Dimension init method.
Support :center in init_with_hash
If center is true, adjusts @left and @top by half of @width and @height respectively.
Updated Rect, Arc and Oval to pass whether :center is set in their opts hash.
Updated Dimension, Arc, Rect and Oval specs.