construct osfclient's file handle

[bpoldrack]

Looked at fixing #24 w/ @mih. Issue was an outdated files property in the special remote.
Problem: Requests to OSF are slow. So, while updating that dict after transfer_store/remove with the appropriate file handler object via osfclient is a correct solution, we should save that update request and build such an instance ourselves. We know everything about that file anyway and 3s for such a request is unbearable in a "real world" scenario.

[bpoldrack]

For reference: #25 has the changes, that this is referring to.