- the `src` folder is now `libmat2`
- the `main.py` script is now `mat2.py`

- Simplify the multiprocessing by using a Pool
- Use some functional () constructions to exit
  with a return code
- Add some tests to prove that we're doing things
  that are working correctly

Since I didn't notice that it was broken
until c5f51345,
it's a good idea to have some tests for this ;)

To be completely honest, BMP have no metadata,
but we still add it, just in case™