Update and document development scripts

[tschaub]

This reworks the npm scripts so that npm start is useful for debugging any failing tests (in a browser).

It also brings back the Coveralls integration. The purpose of this integration is to see how pull requests affect test coverage (without running anything locally). Since the npm test task already generates test coverage output in the console, there is no need to generate an HTML coverage report locally - so the npm run coverage task is only meant to be run on Travis.

I've documented the tasks that are meant to be used during development in the readme.

[tschaub]

@jacobstr @aisaacs - just so we're all on the same page:

• The coveralls integration is there just to review coverage changes for a pull request (like this one. If tests pass for a PR, you can find a link to the coverage report under the CI checks (and npm run coverage is not meant to be run locally).

• The npm start task now runs browser tests "forever" (until SIGINT). The same task starts a file watcher that runs the tests in node on any change. You can see the node test results, the coverage report, and test results from any attached browser in your terminal.
• To debug the tests in a browser, visit the test debug page (http://localhost:9876/debug.html - linked from any attached browser) and open your console. You need to manually reload the debug page on source/test changes.
• To preview the docs (this should be done before publishing), run npm run doc and view the results in build/doc.

Same info is in the readme.