Refactor polyfill, discuss evolution
Refactor polyfill, discuss evolution

* Refactor to give this document the same feel as others.
* Mention PNaCl alongside Emscripten and asm.js.
* Use 'a polyfill' or 'an asm.js polyfill' instead of 'the polyfill', as there can be many polyfills.
* Discuss polyfill evolution, summarizing discussion from #96.

lynnboy/design