Add @php annotation support.

It's now possible to use an annotation to define php version which must
be used to execute a test method.
So, if all test methods depend of the same php version, you can do :

/** @php 5.4 */
class foo extends atoum\test { … }

You can also define php version for one or several methods :

class foo extends atoum\test
{
	/**
	  @php >= 5.3
	  @php < 5.5
	public function testFoo() { … }

	/** @php > 5.4.1 */
	public function testBar() { … }
}