Fix README (utilization of GLPI constructor)

[polkaa]

Mistake is in the utilization of GLPI constructor :

glpi = GLPI(url, token, (user, password))

(README)
while the constructor has the following signature :

def __init__(self, url, app_token, auth_token,
                 item_map=None):

(glpi/glpi.py)

GLPI constructor does not take a tuple of user and password as argument but an authentification token.