fix signature calculation, see http://apiok.ru/wiki/display/api/Authentication+and+authorization
OK API have "without-session" methods (for example notifications.sendSimple), they must be called without access_token param, even empty
fix looking for error_code key in the response, notifications.sendSimple return boolean value
Merge pull request #2 from carbonariy/master

Fix signature calculation and fixes for notifications.* methods request/response