Add src/phpDocumentor/Plugin/Core/Transformer/Writer/ tests

Created as part of testfest 2013

This PR does the following:
* Removes some dead methods in `src/phpDocumentor/Plugin/Core/Transformer/Writer/Xml.php`
* Removes some dead/unused tests
* Fixes an incorrect vsprintf call (see b0ee5a8f3c9b70551b5bde30df4dff50518761c7)
* Adds vfsStream to dev dependencies which allows mocking the file system
* Adds two tests:
  * `tests/unit/phpDocumentor/Plugin/Core/Transformer/Writer/XmlTest.php`
  * `tests/unit/phpDocumentor/Plugin/Core/Transformer/Writer/CheckStyleTest.php`

If you'd like me to omit some things from this PR (in case I removed something I shouldn't have), let me know and I'll revert those changes.