This should close #28

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

Those attacks are DoS-only.

- Rework the testsuite wrt. torrent
- fail at parser's instantiation on corrupted torrent,
  instead of during `get_meta` or `remove_all` call

We're using methods that aren't available in Poppler
below 0.46, so we're checking for this upon import.

This commit is based on ideas from @LogicalDash