Added a basic set notation exercise

In reference to Trello card #468 I wrote a quick exercise (my first) on basic set notation covering union, intersections and differences. I think it is easier to split this topic into a couple of exercises and this seemed the logical starting point.

Unless there is an easier way of handling sets which I can't see I'd guess eventually a utils/sets.js library would be helpful which these functions could be abstracted out into. 
I didn't want to rock the boat first time!

I'd love any comments.