Prepare release 2.7.5 with packet API (#269)

Adds the following API functions:
- API to get signature key IDs for mobile:
	func (msg *PGPMessage) GetHexSignatureKeyIDsJson() []byte

- API to get encryption key IDs for mobile:
	func (msg *PGPMessage) GetHexEncryptionKeyIDsJson() []byte

- API to get the number of key packets in a PGP message:
	func (msg *PGPSplitMessage) GetNumberOfKeyPackets() (int, error)

- API in package `helper` to encrypt a PGP message to an additional key:
	func EncryptPGPMessageToAdditionalKey(messageToModify *crypto.PGPSplitMessage, keyRing *crypto.KeyRing, additionalKey *crypto.KeyRing) error

constants/armor.go

@@ -3,7 +3,7 @@ package constants
 
 // Constants for armored data.
 const (
-	ArmorHeaderVersion = "GopenPGP 2.7.4"
+	ArmorHeaderVersion = "GopenPGP 2.7.5"
 	ArmorHeaderComment = "https://gopenpgp.org"
 	PGPMessageHeader   = "PGP MESSAGE"
 	PGPSignatureHeader = "PGP SIGNATURE"

constants/version.go

@@ -1,3 +1,3 @@
 package constants
 
-const Version = "2.7.4"
+const Version = "2.7.5"