Add getEntity and getEcryptionKeyIDs functions to key and message types respectively (#55)

* add getEntity function to key struct

* add getEncryptionKeyIDs

* add chengelog + bool return in getEncryptionKeyIDs

* fix description

CHANGELOG.md

@@ -16,6 +16,16 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 (msg *PGPMessage) GetArmoredWithCustomHeaders(comment, version string) (string, error)
 ```
 
+- Now one could get key IDs of keys to which the session key is encrypted
+```go
+func (msg *PGPMessage) getEncryptionKeyIDs() ([]uint64, bool)
+```
+
+get x/crypto Entity (components of an OpenPGP key) from Key struct
+```go
+(key *Key) GetEntity() *openpgp.Entity
+```
+
 ### Changed
 - Improved key and message armoring testing