Partially address #13 by adding the `--git <repo>` option to `sandbox
create`. This enables the quick cloning of a remote repository instead
of doing the tedious dance of creating a sandbox, cloning the remote
repo, deleting the sandbox repo them moving the `.git` directory before
doing a `git reset --hard HEAD`.
Additionally, improve the error messages for all `sandbox` commands,
clean up the completions a little, and use a consistent style in the
scripts.
