Update README.

Audit list of shimmed functions against source.

Add a note suggesting the use of `unorm` to shim `String.prototype.normalize`.

Update copyright year.
Describe how to use subclassing functionality of Map/Set/Promise.

Also updated all our test cases to be consistent with the recommended
pattern. No change in functionality; we're just using appropriate ES6
methods (`Object.setPrototypeOf`) and making the `constructor` field
non-enumerable.
Merge pull request #221 from cscott/update-readme

Update README.