Add box_url param to relevant functions; update docs

[nickp60]

See issue #240

This addresses the issue of accessing enterprise box accounts by adding optional args to the box_auth and box_browse functions. I have tried to mirror the behaviour with the client secrets/id where it will look for a BOX_URL environment variable if box_url is not set.

This works on my company's enterprise Box setup, but should probably be checked on someone else's as well.

Happy to discuss further and make any needed changes!