Added function to retrieve users avatar from Mxit.

Returns the avatar as a byte array.

The returned byte array can  be treated as if it was a onfilereceived call with the code:
```
byte[] byteArray;
string mimeType = "";

MXitConnectionModule.RESTConnectionHelper.Instance.GetUserAvatar(tokenReceived.AccessToken, out byteArray, out mimeType  );

//we take the file from Mxit's API call and simulate a file being uploaded 
FileReceived f = new FileReceived(thePlayer.uniqueID, Config.AppConnectedID, "userfile", mimeType, byteArray,DeviceInfo.DefaultDevice, false);
OnFileReceived(f);                            
return;
```