Some API changes to DESFriendManager.
- New:
  - (DESFriend *)addFriendWithoutRequest:(NSString *)theKey
    - Wraps tox_addfriend_norequest(). Returns the added DESFriend on success,
      otherwise returns nil.
  - (DESFriend *)requestWithPublicKey:(NSString *)theKey
    - Behaves similarly to friendWithPublicKey:, except it uses the requests
      array.
- Changed:
  - addFriendWithAddress:message: takes an additional error parameter.
    - On failure, the error's code will be one of DESFriendAddResultCode
    - On success, the error will be nil.
    - The error parameter is optional, so your code will not break.
  - The following methods now return a DESFriend object:
    - (DESFriend *)acceptRequestFromFriend:(DESFriend *)theFriend
    - (DESFriend *)addFriendWithAddress:(NSString *)theKey message:(NSString *)theMessage
    - (DESFriend *)addFriendWithAddress:(NSString *)theKey message:(NSString *)theMessage error:(NSError **)err