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)