Improved composer documentation

Used a more fitting description, added keywords, improved PSR4 autoload (as advised by @stof), added more information to authors, lowered the requirements to avoid conflicts.

@WouterJ and @pyrech : would you like to appear in the author section of `composer.json`?
@gnugat this could even be improved by putting it in ``autoload-dev`` instead. examples don't need to be autoloaded when using the library as a dependency