Add `flight:package` tests

Unit tests for the new `flight:package` generator that will scaffold a
standalone Flight component.
Reorganize app-generator templates

Move all the application-generation templates into a dedicated
directory, in anticipation of the `package` generator.

* Place files that will be shared between application and package
  generators in a 'common' directory.

* Fix issues with the gitignore and Bower configuration. Makes sure that
  Node.js modules are properly ignored.

* Fix app-generator's travis.yml file, so that the generated
  application will configure Travis to install Bower and then use it to
  install the application dependencies before running tests.

* Add a generated CHANGELOG file.
Add `flight:package` templates

All package-specific templates. Includes different Bower installation
directory, and placeholder README.
Implemenet `flight:package` generator
Rework README to include `flight:package`

Involves a larger reorganization of the documentation, and simpler
initial use documentation.