'Called when using a nonce.
This method should return C{True} if the nonce has not been
used before, and store it for a while to make sure nobody
tries to use the same value again.  If the nonce has already
been used or the timestamp is not current, return C{False}.
You may use L{openid.store.nonce.SKEW} for your timestamp window.
@change: In earlier versions, round-trip nonces were used and
a nonce was only valid if it had been previously stored
with C{storeNonce}.  Version 2.0 uses one-way nonces,
requiring a different implementation here that does not
depend on a C{storeNonce} call.  (C{storeNonce} is no
longer part of the interface.)
@param server_url: The URL of the server from which the nonce
originated.
@type server_url: C{str}
@param timestamp: The time that the nonce was created (to the
nearest second), in seconds since January 1 1970 UTC.
@type timestamp: C{int}
@param salt: A random string that makes two nonces from the
same server issued during the same second unique.
@type salt: str
@return: Whether or not the nonce was valid.
@rtype: C{bool}'

def useNonce(self, server_url, timestamp, salt):

    raise NotImplementedError
