Check if we have write access to the target folder of wifi-completion.ba...

...sh before installing it. This comes in handy in case we are installing this wifi module into a virtualenv, in which in most cases we don't want to use superuser permissions.

I've to be honest: I don't know if this is the proper way of achieving my goal... so I've no problem if you reject this PR ;-), I just needed a clean installation via pip into a virtualenv for me.
I did follow the python idiom "It's easier to ask for forgiveness then for permission" in this case. But I guess it's a edge-case here :)