Fixed bug in SheetImage:seSheetSize

was local cw, ch = tw / sizeX, th/ sizeY, resulting in incorrect values
when margin or spacing were >0
Now works with non 0 margin and spacing as follow :
local cw, ch = (tw-(margin or 0)) / sizeX- (spacing or 0),
(th - (margin or 0))/ sizeY - (spacing or 0)
Merge pull request #106 from axboureau/master

Fixed bug in SheetImage:SheetSize