Add docs for new features in mathtext
zorder support in figure: use same sort idiom as in axes
spines and ticks: implement smart bounds
spines: attempt to fix initial placement bug
spines: limit range in example
spines: be clear about meaning of location vertices
spines: default transform is in data units, add set_bounds() call
spines: simplify code
spines: new example
mplot3d: fix axes juggle issue, fix ticks on end of axes range
