Add PGP crypto routines

Signed-off-by: Sven Strickroth <email@cs-ware.de>
Make hash file method text_signature aware

Signed-off-by: Sven Strickroth <email@cs-ware.de>
Integrate new GPG verification in update checker

Signed-off-by: Sven Strickroth <email@cs-ware.de>
Do not link on the info text

Signed-off-by: Sven Strickroth <email@cs-ware.de>
Clear cache only for failed downloads

Signed-off-by: Sven Strickroth <email@cs-ware.de>