Fix powerset

[jankapunkt]

Summary

Power set method has been fixed, also fixed has/add function when Sets are involved. See #50

Behavior

You can either run babel or node (while with node you need to require('./dist/index.js') and enter the following:

``javascript

Set.power(new Set([1, 2, 3]))
Set {
Set { 2, 3 },
Set { 3 },
Set {},
Set { 2 },
Set { 1, 3 },
Set { 1 },
Set { 1, 2 },
Set { 1, 2, 3 } }

or

```javascript
> Set.power(new Set([1, 2, 3, 4 ]))
Set {
  Set { 2, 3, 4 },
  Set { 3, 4 },
  Set { 4 },
  Set {},
  Set { 3 },
  Set { 2, 4 },
  Set { 2 },
  Set { 2, 3 },
  Set { 1, 3, 4 },
  Set { 1, 4 },
  Set { 1 },
  Set { 1, 3 },
  Set { 1, 2, 4 },
  Set { 1, 2 },
  Set { 1, 2, 3 },
  Set { 1, 2, 3, 4 } }

it should also work for sets of sets (according to the tests).

Notes:

One recursion removed and replaced with iteration (should increase perf).

[michaelmior]

I don't really follow what the problem was, but I can confirm that these changes fix things for me :)

[jankapunkt]

@michaelmior fix published in v1.4.2