Properly call the constructor of the super class

The previous statement had no effect. The super classes constructor
sets the .message attribute, so it is safe to drop that statement.

Signed-off-by: Justus Winter <4winter@informatik.uni-hamburg.de>
Verify OpenPGP signatures and display the result

Verify OpenPGP signatures as specified in RFC 3156. Display the result
in the header list above the message.

Signed-off-by: Justus Winter <4winter@informatik.uni-hamburg.de>
Parse and decrypt OpenPGP encrypted data

Parse and decrypt OpenPGP encrypted data as specified by RFC 3156. If
such a message is detected and found to be well-formed, it is
decrypted and any MIME messages found within the plain text are
attached to the original message.

Signed-off-by: Justus Winter <4winter@informatik.uni-hamburg.de>
Remove unused import of logging

Signed-off-by: Justus Winter <4winter@informatik.uni-hamburg.de>
Replace magic value in seek call with the appropriate constant

Signed-off-by: Justus Winter <4winter@informatik.uni-hamburg.de>
Handle missing parameters gracefully

OpenPGP states that the Content-Type header is annotated with
parameters, namely 'protocol' and 'micalg'. Use .get() to retrieve
this values with a default value to handle the case of malformed
(according to RFC 3156) messages gracefully.

Signed-off-by: Justus Winter <4winter@informatik.uni-hamburg.de>
Normalize Content-Type parameters

RFC 2045 specifies that parameter names are case-insensitive, so
normalize them by converting them to their lower case version.

Signed-off-by: Justus Winter <4winter@informatik.uni-hamburg.de>
Move the parameter extraction to its own function

Move the parameter extraction to its own function and generalize it so
it can be reused.

Signed-off-by: Justus Winter <4winter@informatik.uni-hamburg.de>
Fix detection of OpenPGP encrypted data

Fix detection of OpenPGP encrypted data and also check the 'protocol'
parameter.

Signed-off-by: Justus Winter <4winter@informatik.uni-hamburg.de>
Merge branch '0.3.4-feature-gpg-decrypt-616' into testing