add extra config settings

g2o_data_header - the name of the data HTTP header
g2o_sign_header - the name of the signature HTTP header
g2o_hash_function - the hashing function (md5, sha1, sha256)
g2o_version - the expected g2o version
g2o_time_window - the allowed difference between the local time and the
time passed in the g2o data header

the defaults of all settings were defined so that backward compatibility
is retained (X-Akamai-G2O-Auth-Data, X-Akamai-G2O-Auth-Sign, md5, version
3, and 30 seconds window)