Add _.findKey and _.findIndex

As mentioned in #413, these methods are useful internally and complements `_.indexOf` and `_.find`.

If #1582 is merged, `_.findIndex` should be identical to [`lodash.findIndex`](http://lodash.com/docs#findIndex)  and `_.findKey` should be identical to [`lodash.findKey`](http://lodash.com/docs#findKey)
Curious on style thoughts of changing this to

`if (key === void 0 || key === -1) return obj[key]` as explicitly returning undefined seems a bit unnecessary to me