Closes #60

Note: this changeset also ensures that clean.cleaned.docx is removed
up after the pytest is over.

the end result will be the same -- an abort -- but the user will get
to see all the warnings for a particular file, instead of getting them
one at a time.

This makes it easy to get a list of all files that mat2 doesn't know
how to handle, without having to choose -u keep or -u omit.

previously, encountering an unknown member meant that any parser of
this type would abort.

now, the user can set parser.unknown_member_policy to either 'omit' or
'keep' if they don't want the current action of 'abort'

note that this causes pylint to complain about branching depth for
remove_all() because of the nuanced error-handling.  I've disabled
this check.

Pixbuf only supports .jpeg files, not .jpg, so libmat2 looks for such an
extension and converts it if necessary. As this check is case sensitive,
processing .JPG files does not work.

Fixes #47.

Signed-off-by: Antoine Tenart <antoine.tenart@ack.tf>

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