Added autocomplete for fish shell

I create a autocomplete script for fishshell, what about include it in the python package? 
You still need to add one line manually in your config.fish, probably something like
`. /usr/local/lib/python2.7/dist-packages/invewrapper/complete_scripts/complete.fish`
but would be better to have all files together.

Also one benefit is that you can make a new release and update the fish script. That way the users would have an up-to-date version of the autocomplete script just with a `pip install -U`

Of course would be nice to have an autocomplete script for every shell, but I don't have to many experience with this kind of script in other shells.