Add box.com nodes

Some initial work on a box node. The code works but I need to:

- [x] add more logic to handle the behaviour when files already exist on the remote end,
- [ ] make sure a keepalive-like token request occurs if no other requests have happened for a few days ensure the refresh token remains valid (it expires every 60 days otherwise)
- [x] fix msg.event to be more consistent with s3 and dropbox nodes.