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.
Sadly I couldn't use this python module (wifi) at the end and had to go a different way to achieve my goal. So I don't have any env installed to test this again. I'd absolutely no problem if you alter this PR by using the access function. :) Sorry that I don't maintain this PR anymore... it's just to long ago