Removed friendstatus from client-side API, replacing it with alternative functions

Some people on **#tox-dev** complained that there are too many statuses: user status, connection status and friend status. This PR removes *friend status* from client-side API and replaces it with two new functions:

```C
/* Checks friend's connecting status.
 *
 *  return 1 if friend is connected to us (Online).
 *  return 0 if friend is not connected to us (Offline).
 *  return -1 on failure.
 */
int tox_get_friend_connectionstatus(Tox *tox, int friendnumber);

/* Checks if there exists a friend with given friendnumber.
 *
 *  return 1 if friend exists.
 *  return 0 if friend doesn't exist.
 */
int tox_friend_exists(Tox *tox, int friendnumber);
```