change uid vkontakte to screen_name

change "identifier" from "uid" to "screen_name". This field return unique page name (like field nickname, but now field nickname canceled). If screen_name not filled, api return "uid".
So if user fill screen_name api return "my_nickname" or is empty apo return uid but have prefix "id" - "id1235213412".
Update Vkontakte.php
Merge pull request #206 from Asanvery/patch-1

change `displayName` in vkontakte to get `screen_name`