Strict mode support, drop __proto__ usage

Fixes https://github.com/alexgorbatchev/node-browser-builtins/issues/7 and https://github.com/alexgorbatchev/node-browser-builtins/issues/6 .
I also updated util.js `is` checks according to https://github.com/joyent/node/blob/be940b45016380322430411b19a354867f0f9ef2/lib/util.js#L433