modifies + and - for Set class

New behavior: 

```lua 

stooges = Set{"moe","larry","curly"}
stooges = stooges + "shemp"
print(stooges) -- [moe,larry,curly,shemp]

capital = Set{"Constantinople"}
capital = capital + "Istanbul" - "Constantinople"
print(capital) -- Istanbul, not Constantinople now
```

This does not affect union and difference, which should be operations across Sets only imho.