Create a separate fn scope for each $ instance

Defines a per-page $.fn instance that can be modified safely by pages without
causing their content to be retained or unexpected cross context $ operations.
The use of `__proto__`. Jshint does not like but it's in the spec as of ES6. Generally we don't want to use this since there is performance [overhead](http://jsperf.com/constructor-vs-v8-proto).