Switch to mocha/chai

The HTML version covers both the feature tests and the old unit tests (which have been converted to js tests). Run it from `test/runner.html`

To run the node tests do `make test` or if you want to run a specific test or a subset:

```sh
$ mocha --ui tdd -R spec --require test/unit/node-env.js test/unit/runtime/modules.js
```

You can also use `--grep` or `&grep` to run a smaller amount of the tests.

The feature test harness is now the same in both node and html.

Fixes #26

Review URL: https://github.com/google/traceur-compiler/issues/279

Closes #279, closes #26.