Extension API

This is a **WIP** on an extension API for atoum.

It provides an interface to allow users/contributors to write their own extensions, and add some more awesome feature to atoum dynamically.

Some examples:

* https://github.com/jubianchi/atoum-visibility-extension : allows user to test hidden (protected/private) methods. Example : https://github.com/jubianchi/atoum-visibility-extension/blob/master/stdClass.php
* https://github.com/jubianchi/atoum-json-schema-extension : allows user to validate JSON against a schema
* https://github.com/hoaproject/Contributions-Atoum-PraspelExtension : Integrates Praspel in atoum