Unit tests for PSR-4 class loader.

[donquixote]

I added some basic unit tests.
Inspired by https://github.com/symfony/ClassLoader/blob/master/Tests/ClassLoaderTest.php

The symfony test has a number of exotic cases which are poorly documented and are going to be dead weight if we don't know what they do. So I focused on the obvious stuff, and skipped the rest.

[ghost]

PSR4 has now reached a majority at @php-fig even though the 14 day vote period has not completed. So I guess this PR should be finalized now.

[donquixote]

There is a little problem here.
If you run "composer install" on Composer, the vendor/composer/ClassLoader.php will be the one from your system-wide Composer version, and NOT the one from the experimental Composer version we are working on.

The result: composer install + phpunit results in

PHP Fatal error:  Call to undefined method Composer\Autoload\ClassLoader::addPsr4()

We could fix this with a script that replaces the system-generated ClassLoader.php with a copy of the new PSR-4 ClassLoader. I think this is better than to force people to mess with their system-wide composer installation.

[donquixote]

Talked with igorw on IRC #composer-dev.
Solution is that everyone needs to run bin/composer install to try this.
Problem solved.

[donquixote]

Follow-up PR: https://github.com/composer/composer/pull/2459/files

This one includes unit tests and AutoloadGenerator changes.