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 

- xml files are no longer considered harmless
- Factorization of the `remove_all` method for office files
- Explicit whitelist are used
- Blacklist are used to skip files completely
  - Non-blacklisted files are _still cleaned_
  - Unsupported files are still triggering an error

Fedora defines the 'text/xml' mime type for xml files. Adds this mime
type to the harmless parser.

Fixes #36.

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