Add version check between :box command and box shells script.
Protect bin/box from aliases.

Since bin/box is sourced, we need to protect ourselves from aliases in
the user's environment.
Make sure variable in bin/box is local.
Implement directory movement in bin/box.

Move to top of box directory (if it exists) to avoid cases where we
need to remove the current working directory, thereby breaking things.
Upon completion, return to a location as close as we can get to the
original.