Structure recreate support

@milesj in #15 mentioned following:

------------------------------------------
On a side note, it would be nice to have a method like `createStructure()` that can easily create nested files and folders.

	$vfs->createStructure([
		'folder1' => [
			'file1.txt' => 'contents',
			'file2.php' => '<?php',
			'folder2' => []
		]
	]);
------------------------------------------