added PathPatch3D in mplot3d.art3d

Reinier and others,

I just added a PathPatch3D class in mplot3d.art3d.
It is a slight variation of Patch3D that respect the codes the bezier
curve (I believe Patch3D does not).

An example is also added as, with a screenshot below.

It will be appreciated if Reinier of others review the code (the patch
is quite small).
My original intention was to have an option for the labels (including
ticklabels) look as if they are in the particular plane of axes (as
partly demonstrated in my example). But I won't be able to work on it