Add ext-xmlwriter as a Composer requirement

XMLWriter is surprisingly a different extension from xml; and so is not installed on some systems (such as Gentoo).  This commit enforces it to be installed, since it's necessary for the functionality of PHPExcel.
Merge pull request #297 from navarr/composer-xml

Add ext-xmlwriter as a Composer requirement
Update Excel2007.php

Fix output corruption
Merge pull request #328 from sarciszewski/patch-1

Update Excel2007.php