Kudos to Sherry Taylor for reporting this issue ♥️

This should close #74

This allows the user to make use of parser.unknown_member_policy for
archive formats.

At the suggestion of @jvoisin, it also prints a scary warning if the
user explicitly chooses 'keep'.

Example use:

```
$ mat2 -c
Dependencies required for MAT2 0.1.3:
- Cairo: yes
- Exiftool: yes
- GdkPixbuf from PyGobject: yes
- Mutagen: yes
- Poppler from PyGobject: yes
- PyGobject: yes
```

This should close #35

- 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 ;)

- Normalize the case
- Strip \00, \r, space and \n
- Flatten metadata lists
- Add tests for audio files