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.
There is a function in the `os` module called [`access`](https://docs.python.org/2/library/os.html#os.access) which can check if the current user has access to a certain path.  I think it would be cleaner to use that, what do you think?