Feature - Updating Composer schema

This PR quickly updates the Composer schema to:

- Use PSR-4 autoloading
- Use an [`autoload-dev`][composer-autoload-dev] section for autoloading the tests properly
- Use a 2-space indentation
- Alias the release branch for a development version number string
- Rely on a stable version of Klein! 2.1! Woot!

Since I've re-indented the `composer.json` file, here's a convenient link to look at the file diff without whitespace changes: https://github.com/Rican7/Paulus/pull/11/files?w=1

[composer-autoload-dev]: https://getcomposer.org/doc/04-schema.md#autoload-dev