add support for logging frameworks

Currently the PHP SDK does not have any logging support and it relays on using LCB_LOGLEVEL environment variable. Unfortunately in most production environments this cannot be used as the SDK is being used inside php-FPM or FastCGI and we cannot get access to standard error easily if at all.

The python SDK already has this support.

It would be good if the documentation for this feature has examples of setting up logging in php-FPM and FastCGI.