Add @extension annotation support.

Instead of using $this->extension('foo')->isLoaded(), it's now
possible to use an annotation to define mandatory extension.
So, if all test methods depend of some extensions, you can do :

/** @extension mbstring xml phar */
class foo extends atoum\test { … }

You can also define mandatory extensions for one or several methods :

class foo extends atoum\test
{
	/** @extension mbstring */
	public function testFoo() { … }

	/** @extension phar xml */
	public function testBar() { … }
}