Allow specifying optional left and top values to the thumbnail tag which control the centering of the thumbnail.  If non default values are used update the thumbnail name.

--HG--
branch : thumbnail_centering
Merge
Clean up positioning code in thumbnail tag.
added three classes "left", "middle" and "right"

these classes are used by cartridge to use the entire space in checkout process
Merge
Added Sebastian Clemens to AUTHORS.