Updates README introduction.

With the discussion on reddit about Respect/Validation being the most
awesome validation engine and also with a more accurate number of
current rules.
Authors on composer.json are now resumed to a link.

Instead of keeping the file up to date and confusing ourselves with
changes to the file on the version control, the author's not point to
GitHub list of contributors.
Add PHPUnit as a development dependency on composer.json.

We can always use Respect/Foundation, but with that component on
composer.json I think it is easier for new contributors to get things
running on their development environment.