Adding missing MessageKeys serialization

AxolotlKit.podspec

@@ -1,6 +1,6 @@
 Pod::Spec.new do |s|
   s.name         = "AxolotlKit"
-  s.version      = "0.4"
+  s.version      = "0.5"
   s.summary      = "AxolotlKit is a Free implementation of the Axolotl protocol in Objective-C"
   s.homepage     = "https://github.com/WhisperSystems/AxolotlKit"
   s.license      = "GPLv2"

AxolotlKit/Classes/Ratchet/MessageKeys.h

@@ -8,7 +8,7 @@
 
 #import <Foundation/Foundation.h>
 
-@interface MessageKeys : NSObject
+@interface MessageKeys : NSObject <NSSecureCoding>
 
 - (instancetype)initWithCipherKey:(NSData*)cipherKey macKey:(NSData*)macKey iv:(NSData*)data index:(int)index;
 

AxolotlKit/Classes/Ratchet/MessageKeys.m

@@ -8,8 +8,34 @@
 
 #import "MessageKeys.h"
 
+static NSString* const kCoderMessageKeysCipherKey = @"kCoderMessageKeysCipherKey";
+static NSString* const kCoderMessageKeysMacKey    = @"kCoderMessageKeysMacKey";
+static NSString* const kCoderMessageKeysIVKey     = @"kCoderMessageKeysIVKey";
+static NSString* const kCoderMessageKeysIndex     = @"kCoderMessageKeysIndex";
+
+
 @implementation MessageKeys
 
++ (BOOL)supportsSecureCoding{
+    return YES;
+}
+
+- (id)initWithCoder:(NSCoder *)aDecoder{
+    self = [self initWithCipherKey:[aDecoder decodeObjectOfClass:[NSData class] forKey:kCoderMessageKeysCipherKey]
+                            macKey:[aDecoder decodeObjectOfClass:[NSData class] forKey:kCoderMessageKeysMacKey]
+                                iv:[aDecoder decodeObjectOfClass:[NSData class] forKey:kCoderMessageKeysIVKey]
+                             index:[aDecoder decodeIntForKey:kCoderMessageKeysIndex]];
+    return self;
+}
+
+- (void)encodeWithCoder:(NSCoder *)aCoder{
+    [aCoder encodeObject:self.cipherKey forKey:kCoderMessageKeysCipherKey];
+    [aCoder encodeObject:self.macKey forKey:kCoderMessageKeysMacKey];
+    [aCoder encodeObject:self.iv forKey:kCoderMessageKeysIVKey];
+    [aCoder encodeInt:self.index forKey:kCoderMessageKeysIndex];
+}
+
+
 - (instancetype)initWithCipherKey:(NSData*)cipherKey macKey:(NSData*)macKey iv:(NSData *)data index:(int)index{
     self = [super init];