Add support for copying a specific version of a file

[dmkash]

The Box API supports passing in the ID of a file version to copy. We have a need to ensure that if a file is being copied, the original (first) version is the one that is copied, no matter how many versions of the original there may be.

I used a kwarg because that's how name is handled, then added it to the attributes that are sent in the copy request only if an ID was provided.