firstExisting

Created a `firstExisting` function which can be used as a simple way of providing default objects.

Previous discussion on #33, #34, and #48.

Example:

```javascript
    _.firstExisting(null, {});
    //=> {}
```

I placed it into `underscore.util.existential.js` because I couldn't really see a better place to put it, but it might be better off someplace else.