Introduce ES6 Array.prototype.find and Array.prototype.findIndex to poly

Implemented as described by:
* [15.4.4.x Array.prototype.find ( predicate [ , thisArg ] )](https://gist.github.com/rwldrn/5079436)
* [15.4.4.x Array.prototype.findIndex ( predicate [ , thisArg ] )](https://gist.github.com/rwldrn/5079427)