Add GPG signature exception

[michaelweiser]

We need to add an exception for GPG signatures similar to S/MIME (#39) to avoid false positivies.

Suggested by @Jack28.

[michaelweiser]

@Jack28: Can you provide a test case (email with GPG attachment) that reproduces the problem?

[Jack28]

Extracted from peekaboo log for the atteched message:
GPG Signed Message.zip

Sample declared as "application/pgp-signature"

Determined MIME Types: set([u'application/pgp-signature', 'text/plain'])

Adding rule result Ergebnis "unknown" der Regel file_type_on_greylist:97 - Dateityp ist nicht auf der Liste der zu analysierenden Typen (set([u'application/pgp-signature', 'text/plain'])), Analyse wird fortgesetzt: Nein.

Message send: Die Datei "p002" wurde als "unknown" eingestuft

There is no analysis performed for this attachment