Consider caching list data on object

[jtratner]

When you call list() in s3, you get back quite a bit of data:

   {u'ETag': '"<some etag>"',
    u'Key': '<some key>',
    u'LastModified': datetime.datetime(..., tzinfo=tzutc()),
    u'Size': <some size>,
    u'StorageClass': 'GLACIER'}

it might be nice that to just prefill that information into the objects when you call list() or stat() (or give a way to get at raw results from this), so you can, e.g., go get all the glacier objects and restore them. :)

Same idea for Swift objects.

Would help out @kristjaneerik's PR too