Make bikes moveable; separate bike and chain collisions

And refactored and reformatted

I included the Underscore.js library (http://underscorejs.org/), mainly for its `each` method. “for each .. in” is deprecated, but “for … of …” is not widely supported yet. This seems a way around that.
Fix chain overlapping to make it look more realistic
Increase gravity and remove initial push

According to Andy’s suggestions