Logging request/response information for debugging

[Tonkpils]

It would be useful for the SDK to log request/response information or any other data that is useful for debugging. This should also tie into allowing the user to set a custom logger to be used for dumping this request/response information.

[mbryzek]

Just a quick pointer to the logging example at:

https://github.com/flowcommerce/ruby-sdk/blob/master/extensions/logging_http_handler.rb

Leaving open as there is more to do around instrumentation, but hopefully this is a good first step