Add test harness

First try: just run tests in node.

Second pass: some sort of in-browser testing?

Include code coverage.

Directory structure:

```
\src
\test
\fixtures
\spec
index.ts
```
## Tentative plan:

AVA test runner. (TODO it doesn't support in-browser testing yet?)
NYC code coverage (https://github.com/avajs/ava/blob/master/docs/recipes/code-coverage.md)
Ready for TravisCI and [Coveralls](https://coveralls.io/)

Anything ending in .test.ts is a test file. They can live anywhere in src/.