Add categories to --machine output

rev to 3.3.0-dev

* Test travis on oldest supported SDK, bump min SDK to 2.2.0.
* Add missing lint supported by dartfmt --fix
* Fix test

Closes #593.

Use ``` for examples - fixes https://github.com/dart-lang/stagehand/issues/577
Drop package name - fixes https://github.com/dart-lang/stagehand/issues/576

Fixes #571.

Require Dart 2 stable
web-angular: use stable dependencies

Fixes #568

Also:

* Update version to 3.1.1
* Temporary disable Travis job that uses dart_coveralls

Contributes to #561.

This will replace https://github.com/dart-lang/stagehand/wiki/Contributing.

Fixes #547

fixes https://github.com/dart-lang/stagehand/issues/550

Dart 2 does not create a .pub directory
`build/` is not created by pub – move it to a new section

Fixes #542