Add options to output debug logging

[klolos]

libs3 currently does not write any logs, making it difficult to troubleshoot errors. This is particularly problematic in the case of libcurl errors, since a lot of libcurl's error codes are translated to S3StatusInternalError without any further info.

It would be very useful if libs3 allowed enabling detailed logs for the progress of requests, e.g., via an S3_set_log_level() function and S3_LOG_LEVEL environment variable. Also, it would be useful to allow enabling libcurl verbose mode (CURLOPT_VERBOSE) without having to rebuild libs3.