uix:Carousel fix slide animation when looping between 2 slides closes #1...

Slide animation when looping between the first and
last slide was broken(blank slide) when using only two
slides as outlined in #1030, this fixes that issue.

I also modified the example at the end of uix/carousel.py
so you can just test like so::

    cd kivy_install_dir/kivy/uix
    python carousel.py

To test the solution::

    git pull
    git checkout 1030
    python carousel.py