Introduce a flag to preserve timestamps of files

[Ahuge]

This PR is in response to issue #532

We are implementing a flag that when activated will record and store ctime, mtime, and atime in S3 as metadata on the object. When pulling files down, if that data exists, we will set it on the files as well.

Linux & Darwin don't appear to allow setting of ctime, however atime and mtime are preserved. Windows does allow us to set ctime, mtime, and atime, however they are actually referred to as CreationTime, LastAccessTime, and LastWriteTime.

Please let me know if you have feedback on this PR. Thanks!