[LIBCLOUD-540] [WIP] Copy object

This is the beginning of a patch for https://issues.apache.org/jira/browse/LIBCLOUD-540. It's been sitting on my machine gathering dust, so I'm submitting the WIP it to make sure I finish it off.
To be consistent with existing APIs, this should probably take a container object, so `src_container` and `dst_container`, right?

This way it's also more flexible if a provider uses id instead of a name as container's unique identifier.