[ClassLoader] A PSR-4 compatible class loader.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        |

This is a PSR-4 compatible class loader that I'd like to contribute to the ClassLoader component. Since PSR-4 is the most recent FIG standard for an autoloader, I thought a compatible loader should be part of a feature-complete ClassLoader component.

See: https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-4-autoloader.md

PSR-4 does neither replace PSR-0, nor are those standards 100% compatible. This is why I implemented the standard as a new class.

If you decide that my PR is worth merging, I would also provide a PR for symfony-docs with a documentation.