Merge pull request #495 from zakkak/master

Fix #494. Start indexing attachments from 1 instead of 0
Improve decrypt failure handling

On failure to decrypt, list the encrypted part as an attachment
Do not handle signature parts after verification

Since signatures are not listed as attachments there is no reason to
handle them after the verification.
Merge pull request #497 from zakkak/failed_decryption

Improve decrypt failure handling
Merge pull request #498 from zakkak/skip_signatures

Do not handle signature parts after verification