Add AWS::S3::Client Bucket Logging Methods

Adds the following methods and associated tests:
AWS::S3::Client#get_bucket_logging
AWS::S3::Client#put_bucket_logging

Also adds a method to the Nokogiri Sax Handler to handle comments
in XML by ignoring them.
Merge pull request #337 from aws/s3-logging

Add AWS::S3::Client Bucket Logging Methods