Make Map/Set subclassable; support `iterable` argument to constructor.

Use the same technique added for `Promise` to kludge in support for
ES6-style construction and subclassing, factoring out some helper
methods in the process.  Add support for the `iterable` argument to
Map and Set.
Merge pull request #218 from cscott/subclassable-map

Make Map/Set subclassable; support `iterable` argument to constructor.
Update CHANGELOG.
Make sure Object.keys does not throw.

Per https://people.mozilla.org/~jorendorff/es6-draft.html#sec-object.keys - Object.keys should now accept non-objects, and coerce them to objects.
Make sure null and undefined throw
Merge pull request #220 from ljharb/object_keys_no_throw

Make sure Object.keys does not throw.
Update CHANGELOG to include PR #220.
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.
Link to related `unorm` and `prfun` packages.

We already mention `unorm` further down in the README, but make a
reference up top in the 'installation' section as well.
Improve testling coverage.

Fix a `Object.setPrototypeOf` test that was missing an `it` clause, leading
to confusing output on console (Opera 12, IE).

Add promises tests to `testling.html`; define `assert` in `test_helpers.js`.
Workaround Object.setPrototypeOf(..., null) bug in Opera.

We add an extra link to the prototype chain to workaround the issue.
Workaround buggy Promise implementation in Chrome 33.
Merge pull request #224 from cscott/fix-chrome-promises

Workaround buggy Promise implementation in Chrome 33.
Merge pull request #222 from cscott/testling-1

Improve testling coverage.
Merge pull request #223 from cscott/opera-fix

Workaround Object.setPrototypeOf(..., null) bug in Opera.
Adding node 0.4, 0.6, and 0.11
Fix test suite incompatibility with Chrome canary.

Assigning to `constructor` is forbidden, apparently.
On IE <= 10, Object.setPrototypeOf is impossible to shim.

Update test suite so that we don't test functions we can't provide.