Refactored classes, added an objects.py to define and control object primitives, updated demo.py.

From the readme, this describes the current state:

Interactive canvas for editing 3D geometry, using matplotlib.pyplot

This is a demo of xyz_canvas (pre-release, V0.2.1), a Python library to add, edit, and connect 3D wire-frame objects
using only Matplotlib. The idea is that this will be called by code that needs the
user to define / edit these objects in 3D space.

Currently, only lines are supported but the plan is to include at least lines, rectangles,
arcs (circles) and helices. Shape types are seleced via the buttons next to the geometry display.
The clear button is implemented, as is Exit & Close and List, but the others are placeholders.

To add a line, click two points within the axis space (note that no feedback is given for the first click).

The view may be rotated at any time by clicking and dragging just oustide the axis space.

Currently, endpoints / vertices are pinned to the 'closest' backplane (shaded & gridded).
Methods to move vertices into general 3D space by clicking and typing co-ordinates,
snapping to a 3D grid / other objects, will be added soon.