proxy-polyfill

on the beginning I used getters/setters but I switched to Proxy because it allowed for simpler implementation and more flexibility. I may consider to switch back into getters/setters, if support of Proxy is really a problem (although getters/setters have its limitations). There is also Proxy polyfill, although it has the same limitations getters/setters have ("properties you want to proxy must be known at creation time"): https://github.com/GoogleChrome/proxy-polyfill