Two new API functions

- ``int tox_get_friend_address(Tox *tox, int32_t friendnumber, uint8_t *friend_address);``  
  Get the friend address of friendnumber, including nospam and checksum
- ``int tox_friend_seen(Tox *tox, int32_t friendnumber);``  
  Returns whether the friend has ever been online

This will make it easier to write custom save formats.