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.