Add a PSR-4 autoloader for modules. Now modules can declare their classes under the SimpleSAML\Module namespace.

This patch also starts using Composer for other dependencies
(i.e. php-openid and xmlseclibs).

Thanks to Boy Baukema for implementing this!

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@3290 44740490-163a-0410-bde0-09ae8108e29a