1. Add `addAssets` API to `package.js`
2. Rename `getSourcesFunc` to `getFiles` in internal code
3. Changed `PackageAPI#sources` to `PackageAPI#files` with a new structure that
has separate objects for assets and sources
4. Added some tests for different error conditions
5. The same file can now be a source and an asset
Nothing ever made full use of the promise of the complexity of the
API (configuration that can change at runtime) anyway.
This includes completely ignoring the $APP_CONFIG variable.
I opened GitHub issues against the only Atmosphere packages that had
dependencies on application-configuration (none of which actually used
the package other than by declaring the dependency).
