add unit tests

[mrwonko]

I'm currently reluctant to push my latest changes because they are extensive and I fear I have introduced regressions. Automatic unit tests that convert a suite of test files and compare the results with the previous version should be relatively simple to add, though I'll have to investigate if Blender can be automated on startup or if it needs to be executed manually as an operator.

This would address #22.

Blender has useful command line arguments like --background and --python, try them out.