Fix various minor issues (#45)

* Update header version to 2.0.0

* Add space to cleartext message armouring

* Fix password encrypted binary files

* Clear key private params in helpers

* Do not unlock key if private key is nil

* Document changes

* Use defer for ClearPrivateKeyParams

helper/sign_attachment.go

@@ -33,6 +33,7 @@ func EncryptSignAttachment(
 	if unlockedKeyObj, err = privateKeyObj.Unlock(passphrase); err != nil {
 		return nil, nil, nil, err
 	}
+	defer unlockedKeyObj.ClearPrivateParams()
 
 	if privateKeyRing, err = crypto.NewKeyRing(unlockedKeyObj); err != nil {
 		return nil, nil, nil, err