Closure asserter test instance

This allows users on PHP 5.3 to do:

```php
<?php

use atoum;

class myTest extends atoum
{
	public function testFoo()
	{		
		$this
			->given($this->newTestedInstance)
			->then
				->exception(function($test) {
					$test->testedInstance->throwingException();
				})
					->isInstanceOf('runtimeException')
					->hasMessage('Random exception message')
					
				// Instead of
			->given(
				$test = $this,
				$this->newTestedInstance
			)
			->then
				->exception(function() use ($test) {
					$test->testedInstance->throwingException();
				})
					->isInstanceOf('runtimeException')
					->hasMessage('Random exception message')
		;
	}
}
```

Bye bye `use`!