Adding Images to Bars in Race

[andresberejnoi]

These are some of the changes I included to be able to add images to the bars as they move (check gif in README.md file). I am creating a pull request so that other people can take a look.

Right now it is not very generalizable and things could move much faster with someone who actually knows how Matplotlib works. The only changes with respect to the original project are in _bar_chart_race.py and in the README.md file to explain my goal. I included 3 additional parameters to the initializer class plus a docstring at the end.