Extrusion and clyinder exports

This pull request exposes code that essentially already exists within pyPolyCSG. I'm not really sure why they aren't exposed, and I imagine there is a good reason, I'm just curious to know what it is. 

the make_cylinder method is included simply to match the code processes which are already in place for other primitives. 

    p = polyhedron()
    p.make_box() #kosher!
    p.make_cylinder() #method not found previously