Issue116

I believe this should be somewhat stable now.
This patch allows for:
* Recursive module calls: https://github.com/openscad/openscad/blob/issue116/testdata/scad/features/module-recursion.scad
* Cascading child calls: https://github.com/openscad/openscad/blob/issue116/testdata/scad/features/child-child-test.scad

Needs more testing.
Known issue: recursion-test fails (infinite recursion): https://github.com/openscad/openscad/blob/issue116/testdata/scad/misc/recursion-tests.scad