Start group chats.
Add group chat support.
API differences:
- New public class DESGroupChat represents group chat invitations.
  Properties:
  - DESFriendManager *owner
  - DESFriend *inviter
  - NSString *publicKey
  No public methods.
- Changed class DESFriendManager:
  - New properties:
    - NSArray *chatContexts
    - NSArray *groupRequests
  - New methods:
    - (id<DESChatContext>)createGroupChatWithName:(NSString *)aName
    - (id<DESChatContext>)joinGroupChat:(DESGroupChat *)grp
    - (void)rejectGroupChatInvitation:(DESGroupChat *)grp
    - (void)removeGroupChat:(id<DESChatContext>)groupChat
- Changed protocol <DESChatContext>:
  - New methods:
    - (DESContextType)type
- New constants:
  - DESChatContextArrayDidChangeNotification
    - Posted on the main thread when a chat context is created or deleted.
  - DESGroupRequestArrayDidChangeNotification
    - Posted on the main thread when a group chat invite is received or deleted.
  - DESArrayObjectKey
    - Directly replaces DESArrayFriendKey and DESArrayChatContextKey.
  - Enum DESContextType
    - DESContextTypeGroupChat
    - DESContextTypeOneToOne
- Deprecated constants:
  - DESArrayFriendKey
  - DESArrayChatContextKey