move tests to '/test/app' and '/test/ext' so that pub finds them both, and make the libraries 'chrome/app.dart' and 'chrome/ext.dart'

Also:
- cleaned up .gitignore with global rule for 'dart.js',
'dart_interop.js', and 'interop.js'; no need to enumerate these for each
entrypoint.
- CHANGELOG : added not about the addition of the extension API
- README : updated to show the correct import for packaged apps and
extensions