Add support for serializer setting in redis.ini.

Hi there

I think it makes sense to configure the serializer system wide in the redis.ini file. This can still be changed calling $redis->setOption(Redis::OPT_SERIALIZER, Redis::SERIALIZER_XYZ); in the code.

I decided to keep the INI-setting readable and defined new constants containing the possible string values for this setting.

Regards,
Michael