First pass at the reference implementation in ES6.

A few smaller files were completely converted to idiomatic ES6, but for most I did the minimal amount of work necessary to get the tests passing. They will be converted in subsequent commits.
Add reference implementation README.
Rename test/lib/ to test/utils/.

It was getting confusing with lib/.
Make test/utils/ idiomatic ES6.

Also fixed up some minor terminology issues that were left over from renames.
Make the ReadableStream implementation idiomatic ES6.

This included a number of small semantic fixes to more closely follow the spec, e.g. the getters are now on the prototype.
Convert the WritableStream implementation into idiomatic ES6.
Convert tests into idiomatic ES6.

Also do a general pass for readability, renamings, style, etc.