ENH: Generalize plots to 2D/3D
ENH: Generalize plots to 2D/3D

- added make_axes3d that makes a 3d axes object
- added _plot that wraps plt.plot with the same call signature for 2D
and 3D.
- added _set_labels to set all axis labels according to a string pattern
- added scatter and scatter3d as easy examples
- rewrote plot_traj for use in 2D/3D, added plot_traj3d
- rewrote docstrings of plot_traj
- added check if axes is 3d at make_axes and at annotate

daniorerio/trackpy,daniorerio/trackpy