Remove ES6 requirement

[dafky2000]

Support a more diverse set of browsers

[dafky2000]

Need to consider changing a few code choices... When initially written I had in mind only supporting ES6, but this library would be much more useful for ES5 where Web Workers are not available.

Remove/Replace/Polyfill:

• Map()

https://github.com/medikoo/es6-map - just wish it was O(1)

• let, const

[dafky2000]

Note to self: Web Workers are not a Javascript standard, they are an HTML5 standard... That have actually been supported for quite a while by the various browser vendors... Even IE10 (2012) supported Web Workers.