Store and retrieve creation time in `x-amz-meta-date` header

This is a tentative to store and retrieve creation time to make commands
like rsync work correctly. However, this doesn't seem to work as
expected. The date in `x-amz-meta-date` doesn't seem to be read back.

This is a tentative to fix #68.

Just to be clear: this PR doesn't work. This is just to help moving the discussion a bit.