Remove __proto__ use from zepto.

This addresses #970 removing the need to use `__proto__` by relying on a technique that more closely resembles jQuery. This also avoids [emerging ES6 issues](https://bugs.ecmascript.org/show_bug.cgi?id=3256) and removes the need for an IE<11 fallback.

The gzipped size remains the same and there looks to be [nice perf wins](http://jsperf.com/zepto-vs-jquery-selectors/35#chart=bar) too.
Why do we assign `zepto.Z.prototype` when we never use `new zepto.Z`? I know we did before, but we could quit doing that.