'Calculate the flickr signature for a set of params.
data -- a hash of all the params and values to be hashed, e.g.
{"api_key":"AAAA", "auth_token":"TTTT"}'

def __sign(self, data):

    dataName = self.secret
    keys = data.keys()
    keys.sort()
    for a in keys:
        dataName += (a + data[a])
    hash = md5.new()
    hash.update(dataName)
    return hash.hexdigest()
