Batman.SetMapping

I have need of an observable version of `Set::mapToProperty`, so I implemented this locally and thought I'd see if others were interested! In short:

```coffee
myPropertySetMapping = mySet.mappedTo('myProperty')
```

returns a new Set-like object that `get`s "myProperty" from the objects in `mySet`. When items are added, removed, or modified in `mySet`, the set mapping is updated. It doesn't contain duplicates. More in the docs and tests!

Anybody else interested?