memoizing etag, content-length and last-modified from list request on s3object
adding tests for etag, content_length and last_modified memoization on s3object
parsing data when memoizing content_length and last_modified on s3::object_collection
parsing data when memoizing content_length and last_modified on s3::object_collection
adding configuration for s3 to cache object attributes from ListBucket
last_modified is already a Time instance
Merge pull request #610 from bzanchet/master

memoizes data available on the ListBucket request