Reorders axes in SVG output. Fix #145

Currently pygal renders the x-axis before the y-axis, but this means
that because of the painters algorithm, the horizontal guide lines of
the y-axis are above the y-axis line (which is actually an x-axis
guide). If you have custom css that changes the colours of horizontal
guide lines, for example to hide them until hovered over, they create
visual gaps in the y-axis.