reformat code a bit

looks more nice, now
move some code to a util module

this code has been hard to read. The logic is now abstracted behind some
more readable method names provided by a util module.
add test to detect signed pgp mails to utils

this will be needed to support pgp signed mails
hide application/pgp-signature attachments

pgp signatures are not very interesting for the user. These can be
hidden. Some visible element, indicating the mail is signed, would be
useful, though.